Improvements in and relating to data centre cooling

Abstract

Cooling unit to provide a data centre with cooled liquid coolant, comprising: an air inlet 105 receiving ambient air from outside the data centre building; at least one adiabatic cooling unit 119 for

Description

Field of the Invention The present invention concerns liquid cooling systems for data centres. More particularly, but not exclusively, this invention concerns cooling units for data centres, capable of providing liquid cooling. Additionally, this invention concerns hybrid cooling units for data centres, capable of providing both air cooling and liquid cooling. The invention further concerns methods of providing or increasing liquid cooling capacity in a data centre, and an air handler specifically configured for convenient and efficient conversion from entirely air cooling to partial or full liquid cooling. Background of the Invention A data centre is understood to be a large group of networked computer servers, typically provided in a dedicated space within a building. For example, a modern data centre may include high density, high capacity IT systems in a specialist, dedicated space having a carefully controlled environment. A state of the art data centre may include multiple data halls each containing hundreds or even thousands of computer servers, typically contained in racks arranged in rows. In general, each server is provided with electrical power and a network connection. During operation, components of servers typically generate heat, which should be dissipated to avoid overheating and damage to equipment. Organisations, and also individuals, treat data centres as mission-critical facilities, meaning that even short periods of downtime due to equipment malfunction is not tolerated. With the increase in demand for 'cloud' computing services, there is a need not only for additional data centre facilities, but also higher capacity facilities. Alongside increases in computing power of individual servers, the number of servers in each rack, and the number of racks in each data hall increases with each new generation of data centres. Various measures have been used to compare and contrast modern data centres, including the number of servers, building area, and power consumption. According to Data Centre Magazine, a hyperscale data centre should exceed 5,000 servers and 10,000 square feet, and the three largest data centre facilities in the world have total areas exceeding 7 million square feet. Hyperscale data centres often include multiple data halls, each being a single continuous space for accommodating servers. An individual data hall may include IT servers having a total power consumption of 3 MW or more. Racks, or cabinets, housing servers are typically arranged in rows separated by personnel aisles. In order to make efficient use of space, racks have become larger, and aisles longer. For example, a common modern rack design is capable of holding 42 servers stacked one on top the other (a '42U' rack). The dimensions of server racks have become highly standardised, particularly in terms of width. Racks are typically 600mm wide, and a 42U rack may have a height of about 2300 mm, a depth of 1050mm. A single row of racks in the data hall of a Hyperscale data centre may be made up of 22 or more racks, with the data hall accommodating 14 or more rows. Such a high concentration of servers in a relatively small area can present challenges in data centre cooling, especially as increased server power can result in increased heat generation during operation. Servers used for machine learning / artificial intelligence, for example, often generate more heat and thus require greater cooling. Cooling management is considered especially important in modern data centres, where there is a continuing drive for lower PUE (power usage effectiveness) values. PUE is the ratio of total data centre power consumption to server power consumption, and so minimising energy use for equipment cooling is often a high priority. WO2010139921 (Bripco BVBA) describes an especially energy efficient layout for an air cooled data centre, in which the data centre is subdivided into segregated hot and cold zones, including alternating hot and cold aisles separated by rows of server cabinets. Each cold aisle is supplied with cooling air from a cold air corridor, that also functions as a personnel access corridor, that leads from an air handling unit. It has been found that liquid cooling (i.e. using a liquid fluid to transport heat away from servers) can offer increased cooling capacity and / or improved efficiency as compared to air cooling (i.e. using a gaseous fluid to transport heat away from servers), at least in part due to the higher heat capacity of liquids as compared to gases. Liquid cooling systems may utilise servers configured to receive a flow of cooling liquid that passes through conduits embedded in or close to server components requiring cooling. For example, some servers may include chips having integrated microfluidic channels allowing heat to be extracted from targeted areas by fluid flowing through the channels, while others may feature cold-plate heat exchangers immediately adjacent chips (the heat exchangers receiving cooling fluid). Such systems may be referred to as 'direct-to-chip' liquid cooling systems, for example utilising 'direct-to-chip' servers. Other systems may utilise racks having air-to-liquid heat exchangers, thus allowing cooling liquid to be used to cool air in close proximity to severs, which are themselves cooled by the cooling air. Other arrangements may also be envisaged. A feature common to all such arrangements is that a cooled liquid cooling fluid is supplied to, and warmed liquid cooling fluid removed from, the servers or their immediate surroundings. In certain arrangements, liquid cooling fluid may flow in a closed loop extending between a cooling unit (which is configured to receive warmed fluid, cool it down to a suitable temperature, and then provide the cooled fluid for re-use) and a heat exchange unit (which is configured to receive cooled fluid, use the fluid to provide cooling, and then return warmed fluid). It will be appreciated that the heat exchange unit may, for example, be configured for liquid-to-air exchange, or liquid-to-chip heat exchange. As well as a need for new facilities offering higher cooling capacity, there is a demand for facilities capable of being adapted from air cooling to liquid cooling, and for systems suitable for retro-fitting liquid cooling to existing air-cooled facilities. In certain facilities, there may be an ongoing requirement for both air cooling and liquid cooling. For example, a data centre may have one or more areas with conventional servers which may be cooled efficiently and effectively using air cooling, and one or more areas with high-power severs having a greater demand for cooling. Additionally or alternatively, there may be other items of electrical equipment, and / or other server components, that require at least some degree of air cooling, even if all servers in a data centre a configured for liquid cooling. For example, electrical power supply and distribution components may generate heat, and thus require some supply of cooling air to avoid excessive temperatures in the data centre particularly in personnel areas. Thus, in many facilities, it may be desirable to provide both air cooling and liquid cooling. A data centre having a combination of liquid cooling and air cooling (with mechanical air conditioning for air cooling) is disclosed in US2005235671A1 (Hewlett-Packard). The arrangement disclosed in that document allows for the data centre to transition from air cooling to liquid cooling by installing liquid cooled racks and coupling those racks to existing liquid cooling infrastructure. A similar system, with built-in redundant plumbing to allow convenient switching from air cooling to liquid cooling, is disclosed in US2006065000A1 (Hewlett-Packard). Systems for distributing liquid coolant in a data centre are disclosed in US7724518B1 (Exaflop) and EP3800971A1 (Equinix). A liquid cooling system for a multi-level data centre is disclosed in US2021185847A1 (Baidu USA). In that system, fresh air from a common intake is used to cool a cooling liquid that circulates through all data levels. A server rack with built-in liquid cooling manifolds is disclosed in US2021100134A1 (Baidu USA). Examples of liquid cooling server racks are disclosed in US2009133866A1 (IBM). The hybrid device receives facility coolant from an external source, and uses that coolant in a liquid-to-liquid heat exchanger and in an air-to-liquid heat exchanger to supply both cooling air and cooling liquid to servers. Another system that provides both air cooling and liquid cooling to severs is disclosed in US2023397375 Al (Hebei Qinhuai Data Co). A hybrid cooling unit is disclosed in US2023273658A1 (Nvidia Corp). The unit comprises an evaporative cooler for cooling ambient air for use as cooling air in the data centre, with cooled liquid exiting the evaporative cooler also being used to for liquid cooling. A hybrid-cooled data hall is disclosed in US2022078945A1 (Baidu USA), in which fresh air is supplied to a data hall and an overhead cooling air plenum, wherein liquid cooling loops provide heat exchange between server racks and the overhead cooling air planum thus providing racks with both air and liquid cooling. There remains a need for a data centre liquid cooling system offering high efficiency and flexibility. The present invention seeks to mitigate various problems of the prior art. Alternatively or additionally, the present invention seeks to provide an improved data centre cooling system, and method of cooling IT equipment in a data centre. Summary of the Invention The present invention provides, according to a first aspect, a cooling unit for a data centre, for example in a data centre building. Optionally the cooling unit is a cooling unit as described in the claims. Optionally, the cooling unit is a hybrid cooling unit, for example configured to provide cooling air suitable for cooling items of electronic equipment (such as servers) in the data centre, and cooled liquid coolant (also referred to as cooled liquid cooling fluid) suitable for use in cooling items of electronic equipment (such as servers) in the data centre. Preferably, the liquid coolant is suitable for use in 'direct-to-chip' cooling systems, for example wherein the cooling unit is configured to provide cooled cooling liquid suitable for use with 'direct-to-chip' servers. It will be appreciated that such cooled cooling liquid may be suitable if it meets various physical requirements specified by the server manufacturer, such as temperature, viscosity and / or heat capacity. Optionally, the cooling unit comprises an ambient air inlet for receiving ambient air, such as ambient air from outside the data centre building. Optionally the ambient air inlet is an adjustable inlet, such as an adjustable inlet configured to control the amount of ambient air admitted, e.g. into the data centre building. It will be understood that the cooling unit may, for example, be located partially or entirely inside or outside the data centre building. Optionally, the cooling unit comprises at least one adiabatic cooling unit for cooling ambient air to produce cooling air, such as an evaporative cooling unit. It will be understood that an evaporative cooling unit cools air by allowing a liquid, e.g. water, to evaporate into the air stream. Examples of suitable adiabatic cooling units include sprayers and wetted matrix cooling units, preferably the adiabatic cooling unit comprises a wetted matrix cooling unit. Suitable wetted matrix cooling units are described in WO2011 / 148175A2 (Bripco BVBA) and WO2016 / 193153A1 (Bripco BVBA), the contents of which are incorporated herein by reference. It has been found that adiabatic cooling systems provide particularly efficient and effective treatment of air, especially ambient air, in order to provide cooling air meeting predetermined conditions such as falling within and acceptable temperature and / or humidity range. Optionally, the cooling unit comprises at least one liquid-to-air heat exchanger configured to receive warm liquid coolant from the data centre and at least a portion of the cooling air, and exchange heat from the warm liquid coolant to the second portion of the cooling air, thereby providing cooled liquid coolant for the data centre and warm exhaust air. Such warm exhaust air may also be referred to as waste warm air. Optionally, the Iiquid-to-air heat exchanger is arranged for contact with cooling air. For example, the liquid-to-air heat exchanger optionally comprises at least one coil arranged for contact with cooling air, such as an array of cooling coils for receiving cooling fluid to be cooled. Preferably, such cooling coils are configured to cool cooling liquid circulating inside the coils by transfer of heat to air outside the coils. Optionally, the cooling unit comprises one or more pumps circulating the liquid coolant through the at least one liquid-to-air heat exchanger, for example one or more pumps configured to circulate cooling liquid through said cooling coils. Optionally, the cooling unit comprises a warm air outlet for expelling at least a portion of the warm exhaust air, optionally wherein the warm air outlet is in fluid communication with air outside the data centre building. Optionally, the warm air outlet is positioned and configured to allow said warm exhaust air to exit the data centre building. Optionally, the warm air outlet is an adjustable outlet, such as an adjustable outlet operable to control the amount of said warm exhaust air exhausted from the data centre building. Optionally, the data centre comprises an air mixing chamber for receiving (and preferably mixing) ambient air received via the ambient air inlet and one or more of (i) at least a portion of the warm exhaust air, and (ii) warm air from the data centre (such as warm return air). It will be appreciated that the data centre itself may be source of warm air, for example if the data centre is also configured for air cooling in at least one area. In such an arrangement, it may be that all of the warm exhaust air from the liquid to air heat exchanger is exhausted to the outside of the data centre building. Optionally, the mixing chamber receives (and preferably mixes) said ambient air and at least a portion of the warm exhaust air. Optionally, the air mixing chamber is located between the ambient air inlet and the adiabatic cooling unit. Optionally, the cooling unit comprises at least one ambient airflow path along which air comprising ambient air is transported, the ambient airflow path extending from the ambient air inlet. Optionally, the mixing chamber is located on the ambient airflow path at a position upstream of the position of the adiabatic cooling unit. It will be understood that terms 'upstream' and 'downstream' refer to relative positions along an airflow path in the direction of average airflow (which, in the case of the ambient airflow path, is away from the ambient air inlet, e.g. towards the warm air outlet). Optionally, the cooling unit comprises a recycle inlet for conveying said warm air (such as said at least a portion of the warm exhaust air, and / or warm air from the data centre) into the mixing chamber. Preferably, the recycle inlet is an adjustable recycle inlet, for example operable to control the amount of warm exhaust air admitted into the mixing chamber. It has been found that such a mixing chamber arrangement can help to avoid excessively cold ambient air being contacted with equipment in the cooling unit, for example when ambient air temperature is low. Optionally, the cooling unit comprises at least one fan, such as a plurality of fans, for transporting air through the cooling unit. For example, the cooling unit optionally comprises at least one fan, such as a plurality of fans, for transporting air along the ambient airflow path. Optionally, at least one fan, such as a plurality of fans, is configured and / or operable to transport ambient air through the cooling unit from the ambient air inlet to the warm air outlet. Optionally, such fans are located along the ambient airflow path at a position downstream of the adiabatic cooling unit. Optionally, the cooling unit (preferably a hybrid cooling unit) is configured for direct air cooling. It will be understood that in a direct air cooling system, cooling air provided to electronic equipment may comprise, or even consist of, ambient air. Thus, ambient air is used directly for cooling. Thus, the cooling unit is optionally configured so that, during operation, the supply air at least comprises ambient air. It will be understood that such ambient air may be treated, for example to adjust temperature and / or humidity, and still referred to as ambient air. In other words, the term 'ambient air' refers to air that originates from the ambient environment outside the data centre. Examples of direct air cooling systems for data centres are disclosed in WO2011 / 148175A2 (Bripco BVBA) and WO2010 / 139921A1 (Bripco BVBA), the contents of which are incorporated herein by reference. It has been found that direct air cooling can provide particularly efficient and effective cooling, particularly during periods where ambient air is suitable for use as cooling air without adjustment of its temperature and humidity. Optionally, the cooling unit is configured to provide a first portion of cooling air to the liquid-to-air heat exchanger, and to provide a second portion of cooling air as the supply air for the data centre. It will be understood that the first portion is separate to the second portion. Optionally, the cooling air consists of a first portion and a second portion. Preferably, the first portion of cooling air is segregated from the second portion of cooling air by an air barrier, for example so that the second portion of cooling air does not come into contact with the liquid-to-air heat exchanger and the first portion of cooling air is not used as supply air. It has been found that providing separate portions of cooling air can help to improve control of supply air temperature, for example by avoiding undue heating due to contact with the liquid-to-air heat exchanger. Optionally, the mixing chamber is configured to receive said ambient air received via the ambient air inlet, at least a portion of said warm exhaust air, and at least a portion of warm return air from the data centre. It will be understood that warm return air is air that has been used for cooling electronic equipment in the data centre. Thus, supply air used for air-cooling electronic equipment becomes warm return air. Optionally, the cooling unit comprises a return inlet for conveying warm return air into the mixing chamber, Optionally, the return inlet is an adjustable return inlet, for example operable to control the amount of warm return air admitted into the mixing chamber. Optionally, the cooling unit (preferably a hybrid cooling unit) is configured for indirect air cooling. It will be understood that in an indirect air cooling system, supply air (provided to electronic equipment as cooling air) generally consists of warm return air cooled by contact with a heat exchanger. Such a heat exchanger transfers heat from the warm return air to another stream of air, for example a stream of air comprising, or consisting of, ambient air from outside the data centre building. It has been found that indirect air cooling units may be particularly effective when ambient air quality is not deemed suitable for direct air cooling. An example of an indirect air cooling system for a data centre is disclosed in WO2016 / 207323A1 (Bripco BVBA). Optionally, the cooling unit comprises at least one air-to-air heat exchanger to provide indirect cooling. Optionally, the at least one air-to-air heat exchanger is configured to receive warm return air from the data centre and a first portion of the cooling air, and exchange heat from the warm return air to said first portion of the cooling air, thereby providing supply air for the data centre and additional warm exhaust air. Optionally, the cooling unit is configured to provide a first portion of cooling air to the liquid-to-air heat exchanger, and to provide a second portion of cooling air to the air-to-air heat exchanger. It will be understood that the first portion is separate to the second portion. Optionally, the cooling air consists of a first portion and a second portion. Preferably, the first portion of cooling air is segregated from the second portion of cooling air by an air barrier so that the second portion of cooling air does not come into contact with the liquid-to-air heat exchanger and the first portion of cooling air is not contacted with the air-to-air heat exchanger. It has been found that providing separate portions of cooling air can help to improve control of supply air temperature, for example by avoiding undue heating due to contact of the cooling air both with the liquid-to-air heat exchanger and with the air-to-air heat exchanger. Optionally, the air-to-air heat exchanger comprises a heat exchanger unit (such as a plate, heat wheel or heat tube, preferably a heat tube) comprising a first section arranged for contact with the cooling air and a section arranged for contact with the warm return air. Optionally, the cooling unit comprises an internal airflow path segregated from an external airflow path. Optionally, the internal airflow path extends from a return air inlet to a supply air outlet, the return air inlet being configured to receive warm return air from the data centre. Optionally, the external airflow path extends from the ambient air inlet to the warm air outlet. Optionally, the cooling unit comprises a plurality of fans for transporting air along the internal airflow path. Alternatively, the internal airflow path is free from fans (e.g. fans suitable for transporting air along the internal airflow path). Optionally, the cooling unit comprises a plurality of fans operable to transport air from the ambient air inlet to the warm air outlet. Optionally, the cooling unit comprises a supply air outlet arranged to convey the supply air to the data centre, optionally wherein the supply air outlet is in fluid communication with one or more cold zones of the data centre, such as one or more cold corridors or cold aisles. Suitable data centre layouts are disclosed in WO2010 / 139921A1 (Bripco BVBA) and pending International PCT Application No. PCT / GB2023 / 053070 (Pripco Ltd), the contents of which are incorporated herein by reference. Optionally, the cooling unit comprises a recycle airflow conduit configured to transport said warm exhaust air to the warm air outlet and to the air mixing chamber (e.g. via a return air inlet). Optionally, the recycle airflow conduit is configured not to receive warm return air (e.g. the recycle airflow conduit is not in fluid communication with any hot zone areas of the data centre, hot zone areas being those areas containing or conveying warm return air). Alternatively, when the cooling unit is a direct air hybrid cooling unit, the recycle airflow conduit is configured to receive warm return air. Optionally, the cooling unit comprises mechanical cooling equipment configured for cooling at least one of the cooling air and the supply air. It will be understood that mechanical cooling systems include direct-expansion (DX) cooling systems. For example comprising at least one evaporator coil for cooling (and optionaIly dehumidifying) air, at least one condenser coil for expelling heat, a pump for circulating refrigerant through the coils, and a compressor for compressing refrigerant prior to re-evaporation. Optionally, when the cooling unit is a direct air hybrid cooling unit, the cooling unit comprises mechanical cooling equipment configured to treat the ambient air, thereby cooling (and optionally dehumidifying) the ambient air. Optionally, when the cooling unit is an indirect air hybrid cooling unit, the cooling unit comprises mechanical cooling equipment configured to treat the return air, for example to treat air transported along the internal airflow path. It has been found that mechanical cooling equipment can be useful if there are periods in which ambient air and adiabatic cooling cannot provide appropriate supply air (e.g. when ambient air is particularly warm and / or damp). Optionally, a hybrid cooling unit is sub-divided by an air barrier into an air cooling section and a liquid cooling section. For example, the air barrier divides ambient airflow through the hybrid cooling unit so that a portion of air from the mixing chamber is utilised for provision of supply air, and a second, separate portion of air from the mixing chamber is used for cooling liquid coolant. It will be understood that such sub-division may be provided at any point along the ambient airflow path, provided that cooling air used in the provision of supply air is kept separate from cooling air used for cooling the liquid coolant. Optionally, the air barrier is positioned immediately downstream of the mixing chamber. It will be understood that the barrier is positioned immediately downstream of the mixing chamber when no equipment for adjusting temperature or humidity, and optionally no equipment for air purification or filtration, is positioned between the mixing chamber and the upstream end of the air barrier. Optionally, the air cooling section is operable to provide the supply air. Optionally, the liquid cooling section is operable to provide the cooled liquid coolant. Optionally, the liquid cooling section and at least a portion of the air cooling section is in fluid communication with the mixing chamber. Thus, the air cooling and liquid cooling sections preferably share a common supply of ambient air. Optionally, the air cooling section and the liquid cooling section each comprise an independently controllable adiabatic cooler operable to treat air received from the mixing chamber. It has been found that such independent control allows cooling to be varied, e.g. depending on the cooling air and cooling liquid demands of the data centre. Optionally, the air cooling section and the liquid cooling section each comprise one or more independently controllable fans. It has been found that such independent control allows airflow to be varied in the sections, e.g. depending on the cooling air and cooling liquid demands of the data centre. Optionally, the air cooling section and the liquid cooling section each comprise one or more independently controllable mechanical cooling units. Optionally, the air cooling section has a maximum ambient airflow rate and the liquid cooling section has a maximum ambient airflow rate. It will be understood that the maximum ambient airflow rate of a section is the rate of ambient airflow through the section when operating at maximum capacity. It will be understood that maximum airflow rate will depend on the dimensions of sections of ambient airflow pathway through each section, and on the total fan capacity. Optionally, the maximum ambient airflow rate of the liquid cooling section is two to six times the maximum ambient airflow rate of the air cooling section. Thus, the hybrid cooling unit is configured so that the liquid cooling section is able to make use of the majority of air from the mixing chamber. Optionally, a hybrid cooling unit is configured to provide 500 kW to 1MW of combined air and liquid cooling capacity, such as 700 kW to 900 kW of combined air and liquid cooling capacity. Additionally or alternatively, the hybrid cooling unit is configured to provide 75 kW to 150 kW air cooling capacity, such as 100 kW to 130 kW air cooling capacity. Additionally or alternatively, the hybrid cooling unit is configured to provide 425 kW to 850 kW of liquid cooling capacity, such as 550 kW to 700 kW of liquid cooling capacity. Additionally or alternatively, the hybrid cooling unit is configured to provide cooling air at a maximum rate of 40 m3 / s to 60 m3 / s, such as 35 m3 / s to 60 m3 / s. Additionally or alternatively, the hybrid cooling unit is configured to provide supply air at a temperature of from 18°C to 32°C and a relative humidity of from 20% to 80%. Additionally or alternatively, the hybrid cooling unit is configured to provide cooled liquid coolant at a temperature of from 25°C to 45°C, such as 30°C to 40°C. Additionally or alternatively, the hybrid cooling unit is configured to operate with a warm coolant return temperature of from 35°C to 50°C, such as 40°C to 50°C. Additionally or alternatively, the hybrid cooling unit is configured to cool liquid coolant by 5°C to 15°C, such as 7.5°C to 12.5°C (i.e. the unit is configured to operate with flow / return temperatures differing by amounts falling in those ranges). It will be understood that cooled liquid coolant may, for example, be suitable for liquid-cooling electronic equipment in a data centre, for example having properties (such as temperature, viscosity and / or thermal capacity) meeting pre-set requirements of liquid-cooled electronic equipment in a data centre. According to a second aspect of the invention, there is provided a method of providing or increasing liquid cooling in a data centre. Optionally, the method comprises converting an air cooling unit to a hybrid cooling unit, or to a liquid cooling unit. It will be understood that, prior to performance of the method, the data centre comprises at least one cooling unit that provides only cooling air (and not cooling liquid) to the data centre. It will be further understood that the data centre may optionally already comprise one or more cooling units providing liquid cooling. Optionally, the method is as described in the claims. Preferably, following performance of the method, the data centre comprises a modified cooling unit configured to supply cooled liquid coolant (also referred to as cooled liquid cooling fluid) suitable for use in cooling items of electronic equipment (such as servers) in the data centre. Optionally, the modified cooling unit is a cooling unit (such as a hybrid cooling unit) according to the first aspect of the invention. Optionally, the modified cooling unit configured only for supply of cooled liquid coolant, and not for supply of cooling. Preferably, the liquid coolant is suitable for use in 'direct-to-chip' cooling systems, for example wherein the cooling system is configured to provide cooled cooling liquid suitable for use with 'direct-to-chip' servers. It will be appreciated that such cooled cooling liquid may be suitable if it meets various physical requirements specified by the server manufacturer, such as temperature, viscosity and / or heat capacity. Optionally, said modified cooling unit is also configured to provide cooling air suitable for cooling items of electronic equipment (such as servers) in the data centre. Optionally, prior to performance of the method, the data centre comprises cooling apparatus configured to provide supply air for air-cooling electronic equipment in the data centre. Preferably, the cooling apparatus comprises an adiabatic cooling unit operable to treat ambient air from outside the data centre to produce cooling air. Optionally, the adiabatic cooling unit is an adiabatic cooling unit as described in relation to the first aspect of the invention (e.g. the adiabatic cooling unit comprises an evaporative cooler, such as a spray or wetted matrix cooling unit). Optionally, the cooling apparatus comprises: one or more fans for transporting air through the cooling apparatus, filters e.g. for filtering ambient air, and / or mechanical cooling equipment for additional cooling (and optionally dehumidification) of air used as supply air. Optionally, the cooling apparatus is configured to provide 500 kW to 1MW of air cooling capacity, such as 700 kW to 900 kW of air cooling capacity. Additionally or alternatively, the cooling apparatus is configured to provide cooling air at a maximum rate of 250 m3 / s to 500 m3 / s, such as 300 m3 / s to 400 m3 / s. Additionally or alternatively, the cooling apparatus is configured to provide supply air at a temperature of from 18°C to 32°C and a relative humidity of from 20% to 80%. Optionally, the method comprises providing at least one liquid-to-air heat exchanger, and configuring the liquid-to-air heat exchanger to receive warm liquid coolant from items of electronic equipment in the data centre. Optionally, the method comprises configuring the data centre to direct to the liquid-to-air heat exchanger at least a portion of the cooling air and at least a portion of said warm liquid coolant so that the liquid-to-air heat exchanger is operable to exchange heat from the warm liquid coolant to said portion of the cooling air and thereby provide warm exhaust air and cool liquid coolant. Optionally, the method comprises configuring the cooling apparatus so that at least a portion of the warm exhaust air can be exhausted out of the data centre, and one or more of (i) at least a portion of the warm exhaust air and (ii) warm air from the data centre, can be combined with ambient air in a mixing chamber (preferably wherein at least a portion of the warm exhaust air can be combined with ambient air in a mixing chamber). Optionally, the mixing chamber is located between an ambient air inlet and the adiabatic cooling apparatus so that the adiabatic cooling apparatus receives ambient air via the mixing chamber. Optionally, the method comprises installing the liquid-to-air heat exchanger in a cooling unit containing the cooling apparatus. Optionally, prior to performance of the method, the cooling apparatus is a direct air cooling apparatus. Alternatively, the cooling apparatus is optionally an indirect air cooling apparatus. Additionally or alternatively, the cooling apparatus optionally comprises at least one air-to-air heat exchanger configured to receive warm return air from the data centre and at least a portion of the cooling air, and exchange heat from the warm return air to said at least a portion of the cooling air, thereby providing cooled supply air for the data centre and further warm exhaust air. Optionally, the method comprises removing from the cooling apparatus at least one said air-to-air heat exchanger and replacing it with at least one said liquid-to-air heat exchanger. Optionally, the method comprises providing in the data centre liquid coolant pipework for transporting liquid coolant to and from electronic equipment requiring liquid cooling. Optionally, the method comprises providing data centre equipment configured to receive cool liquid coolant and produce warm liquid coolant. According to a third aspect, there is provided an air handling unit for a data centre building. Optionally, the air handling unit comprises an ambient air inlet for receiving ambient air from outside the data centre building. Optionally, the air handling unit comprises at least one adiabatic cooling unit for cooling ambient air to produce cooling air, optionally an evaporative cooling unit such as a wetted matrix cooling unit. Optionally, the adiabatic cooling unit is as described in relation to the first aspect of the invention. Optionally, the air handling unit comprises a cooling air outlet for conveying the cooling air to the data centre. Optionally, the air handling unit comprises a plurality of fans for transporting air from the ambient air inlet to the cooling air outlet. Optionally, the air handling unit comprises a heat exchanger space suitable for accommodating at least one liquid-to-air heat exchanger, preferably wherein the space is positioned between the adiabatic cooling unit and the cooling air outlet. For example, the heat exchanger space is sized and configured to accommodate a liquid cooling heat exchanger, for example a liquid cooling heat exchanger as described in relation to the first aspect of the invention. Additionally or alternatively, the liquid cooling heat exchanger is Additionally or alternatively, the cooling unit is configured to provide 425 kW to 850 kW of liquid cooling capacity, such as 550 kW to 700 kW of liquid cooling capacity. Additionally or alternatively, the heat exchanger space is sized and configured to accommodate liquid cooling apparatus suitable for: providing cooled liquid coolant at a temperature of from 25°C to 45°C, such as 30°C to 40°C, operating with a warm coolant return temperature of from 35°C to 50°C, such as 40°C to 50°C, and / or cooling liquid coolant by 5°C to 15°C, such as 7.5°C to 12.5°C. Optionally, the cooling apparatus comprises a warm air outlet for expelling out of the data centre building at least a portion of warm exhaust air from the heat exchanger space. Optionally, the air handling unit comprises an air mixing chamber for receiving ambient air via the ambient air inlet and one or more of (i) at least a portion of warm exhaust air from the heat exchanger space and (ii) warm air from the data centre, preferably wherein the air mixing chamber is located between the ambient air inlet and the adiabatic cooling unit. Optionally, the warm air outlet is closed and the cooling air outlet is open. Optionally, the air handling unit comprises a recycle conduit extending from an exit chamber downstream of the adiabatic cooling unit to the mixing chamber. Optionally, the air handling unit comprises a first recycle opening between the exit chamber and the recycle conduit, and a second recycle opening between the recycle conduit and the mixing chamber, preferably wherein one or both of the first and second recycle openings is closed. It has been found that such an air handling unit is ready for convenient conversion to partial or complete liquid cooling. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a top plan view of a hybrid cooling unit according to a first embodiment of the invention; Figure 2 shows a cut-away side view of the hybrid cooling unit of Figure 1; Figure 3 shows another cut-away side view of the hybrid cooling unit of Figure 1; Figure 4 shows a cut-away side view of a hybrid cooling unit according to a second embodiment of the invention; Figure 5 shows another cut-away side view of the hybrid cooling unit of Figure 4; Figure 6 shows a cut-away end view of the hybrid cooling unit of Figure 4 at point A-A; Figure 7 shows a cut-away end view of the hybrid cooling unit of Figure 5 at point B-B; Figure 8 shows a top plan view of a cooling unit according to a third embodiment of the invention; Figure 9 shows a cut-away side view of the cooling unit of Figure 8; Figure 10 shows a cut-away side view of the cooling unit of Figure 8 following conversion to liquid cooling; and Figure 11 illustrates an flow-chart overview of a method according to a fourth embodiment of the invention. Detailed Description Figure 1 shows a top plan, schematic view of a hybrid cooling unit 101 according to a first embodiment of the invention. The hybrid cooling unit 101 may be positioned, for example, on a side of a data centre building (not shown in Figure 1), although other positions may be envisaged. The hybrid cooling unit is configured to supply a data centre housed within the building with cooling air 102 and cooling liquid 103 (see Figure 2), and to receive from the data centre warmed cooling liquid 104 (see Figure 2) requiring cooling. The cooling unit comprises adjustable ambient air inlet 105 for receiving ambient air 106 from outside the data centre building. The hybrid cooling unit 101 of Figure 1 is configured for direct air cooling, meaning that, depending on the mode of operation, the cooling air 102 may comprise or consist of ambient air 106 from outside the data centre building. The adjustable ambient air inlet 105 can be operated to vary the amount of ambient air admitted into mixing chamber 107, in which it may be mixed with warm air 108 returned from the data centre and / or warm air 109 recycled from the liquid cooling section of the hybrid cooling unit 101. Flow of warm air 108 / 109 into the mixing chamber 107 is controlled by operation of adjustable recycle vents 110 / 111, respectively. It will be understood that warm air 108 returned from the data centre is recycled cooling air 102 that has been used to air-cool items of electronic equipment in the data centre. It will further be understood that warm air 109 is air that has been used to cool liquid coolant in the hybrid cooling unit. The extent to which warm air 108 / 109 is mixed with ambient air 106 may be determined in dependence on the condition of the ambient air. For example, if ambient air temperature is low, warm air 108 / 109 may be mixed with ambient air to avoid excessively cold air being processed in the hybrid cooling unit (which could result in unwanted condensation of moisture on equipment in the cooling unit). Warm air 108 / 109 not returned to the mixing chamber may instead be exhausted from the data centre building through exhaust vents 112 / 113, respectively (exhaust vents 112 / 113 being operable to control the amount of warm air 108 / 109 so exhausted). In the embodiment show in Figure 1, the hybrid cooling unit 101 has an air cooling section through which a first portion of air 114 flows from the mixing chamber 107, and a liquid cooling section through which a second portion of air 115 flows from the mixing chamber. The air cooling section and the liquid cooling section each comprise independent portions of a sequence of items of air treatment / processing equipment, including a filter bank 117a / 117b, mechanical cooling equipment 118a / 118b (including evaporator coils of a direct-expansion cooling system), wetted matrix adiabatic cooling system 119a / b, and fan bank 120a / b. For each item of equipment, components with the suffix 'a' are located in the liquid cooling section, and components with the suffix 'b' are located in the air cooling section. In the embodiment shown in Figure 1, equipment in the liquid cooling section is controllable independently of the corresponding equipment in the air cooling section. Optional mechanical cooling equipment 118a / 118b form parts of a mechanical cooling system also including a pump, a compressor and condensation coils not shown in Figure 1 (condensation coils are typically positioned outside the data centre building, such as on the roof). It will be appreciated that such mechanical cooling equipment may or may not be required, depending on a predetermined specification for temperature of cooled liquid coolant 103 and / or the ambient environment in which the data centre is located. Between the wetted matrix adiabatic cooling system 119a and the fan bank 120a, the liquid cooling section further comprises liquid cooling system equipment 121, including a set of cooling coils configured to receive from the data centre warm liquid coolant 104 and provide to the data centre cooled liquid coolant 103. A pump (not shown in Figure 1) is provided to circulate cooling liquid from the data centre, through the cooling coils of the liquid cooling unit 121, and back to the data centre. The cooling coils act as a liquid-to-air heat exchanger, which in use exchanges heat from warm liquid coolant to the first portion of air 114, thereby cooling the liquid coolant and warming the air 114. In the embodiment shown in Figure 1, the air cooling section and the liquid cooling section are segregated by an air barrier 116 so that the first and second portions of air 114 / 115 are kept separate once they exit the common mixing chamber 107. Alternatively, segregation may be provided through only a portion of the hybrid cooling unit, provided that air is segregated at least for the portion of the hybrid cooling unit downstream of the liquid cooling equipment 121, thereby dividing air into a first portion that, during operation, is warmed by contact with the liquid cooling equipment 121, and a second portion that is not contacted with the liquid cooling equipment 121, the second portion being used as supply air to cool electronic equipment in the data centre. Air exiting the air cooling section may be referred to as 'supply air' 102, and may be directed into the data centre for use in air-cooling of electronic equipment in the data centre. For example, such supply air 102 may be transported along an air supply corridor 122 to cold aisles for contact with air-cooled servers positioned between cold aisles and hot aisles (not shown in Figure 1). Such supply air may cool electronic equipment as it passes through the electronic equipment, thereby becoming warm air 108. Warm air may, for example be returned from hot aisles to an air return corridor 123 and / or to a warm air return plenum, and from there be exhausted out of the data centre building and / or returned to the mixing chamber 107, as described above. Air exiting the liquid cooling section may be referred to as waste warm air 109. Waste warm air may be exhausted from the data centre building, or recycled to the mixing chamber 107 via recycle pathway 124. In the embodiment shown in Figure 1, the recycle pathway 124 extends along one side of the hybrid cooling unit, next to the liquid cooling section. Alternatively or additionally, the recycle pathway 124 may be positioned at the top of the hybrid cooling unit, above the liquid and / or air cooling section. It will also be appreciated that other components could be included in the hybrid cooling unit, such as sound attenuation equipment to reduce fan noise. It will be appreciated that a similar structure may be adopted for a liquid-only cooling unit, for example by omitting the air-cooling elements shown in the embodiment depicted in Figure 1. Figure 2 shows a cut-away side, schematic view of the hybrid cooling unit 101 of Figure 1. The view in Figure 2 is through the liquid cooling section of the hybrid cooling unit. The same reference numerals are used as for Figure 1. Figure 3 shows another cut-away side, schematic view of the hybrid cooling unit of Figure 1. The view in Figure 3 is through the air cooling section of the hybrid cooling unit. The same reference numerals are used as for Figure 1. The liquid cooling and air cooling sections of the hybrid cooling unit 101 of Figures 1-3 is sized and configured to have a ratio of maximum airflow of 3:1 (i.e. the liquid cooling section has a maximum airflow capacity three times that of the air cooling section). It will be understood that various variations of the embodiment shown in Figures 1-3 are envisaged. As noted above, the cooling unit may be configured for liquid cooling only, such as by omitting the air cooling section. By way of another example, mechanical cooling equipment may be omitted from at least the liquid cooling section, or entirely, and air filters may be provided only in the air cooling section (for example filtering only the air that is used for air cooling electronic equipment in the data centre). Other types of adiabatic cooling system may be used, such as a spray system. Fans may be omitted from the air cooling section, for example if individual equipment fans are used to transport cooling air in the data centre. Furthermore, the order and / or positioning of equipment may be varied, provided that the adiabatic cooling unit (and mechanical cooling unit, if present) is positioned between the ambient air inlet and the liquid cooling unit in the liquid cooling section. Yet further, the hybrid cooling unit 101 may optionally comprise a warm air return plenum for receiving warm return air from the data centre (e.g. from hot aisles and / or a warm air return corridor 123), such a plenum positioned above at least the air cooling section of the hybrid cooling unit, for example. In such an embodiment, one or more exhaust vents 112 may form part of the hybrid cooling unit 101. Figure 4 shows a cut-away side, schematic view of a hybrid cooling unit 201 according to a second embodiment of the invention. The view of Figure 4 shows the air cooling section of the hybrid cooling unit 201. The hybrid cooling unit 201 may be positioned, for example, on a side of a data centre building (not shown in Figure 4), although other positions may be envisaged. The hybrid cooling unit is configured to supply a data centre housed within the building with cooling air 202 and cooling liquid 203 (see Figure 7), and to receive from the data centre warmed cooling liquid 204 (see Figure 7) requiring cooling. The cooling unit comprises an ambient air inlet for receiving ambient air 206 from outside the data centre building. The hybrid cooling unit 201 of Figure 1 is configured for indirect air cooling, meaning that the cooling air 202 is made up of return air 208 (that is, air returned from inside the data centre which has been used for cooling data centre electronic equipment) has been (re)cooled. Typically, in a data centre utilising indirect air cooling, air circulates around the data centre in a closed loop extending from the air cooling unit to electronic equipment and back to the air cooling unit (although in some arrangements some ambient air may be admitted to refresh the internal air). The hybrid cooling unit 201 includes an air cooling section separated from a liquid cooling section by an air barrier 216 (see Figures 6 and 7), the air cooling section being divided into an internal airflow path 240 segregated from an external airflow path 241. Two air-to-air heat exchangers 242 are provided, allowing heat to be exchanged from air in the internal airflow path 240 to air in the external airflow path 241. In the embodiment shown in Figure 4, the heat exchangers are heat tube heat exchangers 242, each being a panel comprising a panel of heat tubes. Each heat tube has an upper section 242a that extends into the external airflow path 241 ad a lower section 242b that extends into the internal airflow path 240. In operation, ambient air 206 flowing along the external airflow path 241 cools the upper sections 242a of the heat tubes, thereby warming the air in the external airflow path to generate warm exhaust air 243 that is exhausted from the data centre building. Air is drawn along the external airflow path 241 by external airflow fans 244. Return air 208 flowing along the internal airflow path 240 is cooled by contact with the lower sections 242b of the heat tubes, producing cooled supply air 202. In certain embodiments, the hybrid cooling unit 201 may comprise fans 245 in the internal airflow oath 240 for transporting internal air through the hybrid cooling unit 201, whereas in other embodiments such fans may be omitted, for example if fans of individual items of electronic equipment in the data centre are used to transport internal air. The internal airflow path 240 further comprises a wetted matrix adiabatic cooling system 219b positioned between the ambient air inlet and the heat tube heat exchanger 242. The air cooling section also comprises mechanical cooling equipment, including evaporator coils 246 positioned in the internal airflow path 240 and condenser coils 247 positioned in the external airflow path 241. The mechanical cooling system further comprises a pump and a compressor (not shown in Figure 4) for circulating refrigerant between the evaporator coils 246 and the condenser coils 247. When operated, the mechanical cooling system provides additional cooling for air in the internal airflow path 240, with the resulting heat rejected into the external airflow path 241. Figure 5 shows another cut-away side, schematic view of the hybrid cooling unit 201 shown in Figure 4 - the view of Figure 5 shows the liquid cooling section of the hybrid cooling unit 201. In the embodiment shown in Figure 5, the liquid cooling section of the hybrid cooling unit 201 occupies only the top part of the hybrid cooling unit 201, running alongside the external airflow path 241 of the air cooling section. A wetted matrix adiabatic cooling unit 219b is provided for cooling ambient air 206 prior to contact with a liquid cooling unit 221. In the embodiment shown in Figures 4-7, wetted matrix adiabatic cooling units 219a / 219b are independently controllable. The liquid cooling unit 221 is a liquid-to-air heat exchanger in the form of an array of cooling coils, and in use operates to exchange heat from warm liquid coolant 204, thereby producing cooled liquid coolant 203 and warm air 209 (which exits the data centre building through an exhaust vent). A pump (not shown in Figure 5) is provided to circulate cooling liquid from the data centre, through the cooling coils of the liquid cooling unit 221, and back to the data centre. In operation, ambient air 206 is drawn through the liquid cooling section by fans 220a. In the embodiment shown in Figure 5, no mechanical cooling is provided in the liquid cooling section. Figure 6 shows a cut-away end, schematic view of the hybrid cooling unit 201 of Figure 4. The view in Figure 6 is taken along line A-A as indicated in Figures 4 and 5. The same reference numerals are used as for Figures 4 and 5. Figure 7 shows a cut-away end, schematic view of the hybrid cooling unit 201 of Figure 4. The view in Figure 7 is taken along line B-B as indicated in Figures 4 and 5. The same reference numerals are used as for Figures 4 and 5. The liquid cooling and air cooling sections of the hybrid cooling unit 201 of Figures 4-7 is sized and configured to have a ratio of maximum airflow of 2:1 (i.e. the liquid cooling section has a maximum airflow capacity double that of the air cooling section). It will be understood that various variations of the embodiment shown in Figures 4-7 are envisaged. For example, mechanical cooling equipment may be included in the liquid cooling section, or omitted entirely, and air filters may be provided in one or both of the air cooling and liquid cooling sections. Other types of adiabatic cooling system 219 may be used, such as a spray system. In the embodiment shown in Figures 4-7, the external airflow path 241 of the air cooling section is fully segregated both from the internal airflow path 2410 and the liquid cooling section. Alternatively, the external airflow path 241 may be only partially segregated, or not segregated, from the liquid cooling section. One or more recycle pathways and a mixing chamber may be provided, allowing warm air 243 (external airflow path 241 exhaust air) and / or warm air 209 (liquid cooling section exhaust air) to be mixed with ambient air 206 prior to contact with adiabatic cooling equipment 219. Such a mixing chamber may, for example, be common to the air cooling section and the liquid cooling section. Furthermore, the order and / or positioning of equipment may be varied, provided that the adiabatic cooling unit (and mechanical cooling unit, if present) is positioned between the ambient air inlet and the liquid cooling unit in the liquid cooling section. Figure 8 shows a top plan, schematic view of a cooling unit 301 according to a third embodiment of the invention. The cooling unit 301 is structured and configured for convenient conversion from full air cooling to partial or full liquid cooling. In the configuration shown in Figure 8, the cooling unit is configured to supply a data centre housed within a data centre building with cooling air 302, but not cooling liquid, and to receive from the data centre warm return air 308 (see Figure 9), but not warmed cooling liquid. The cooling unit comprises adjustable ambient air inlet 305 for receiving ambient air 306 from outside the data centre building. The cooling unit 301 of Figure 8 is configured for direct air cooling, meaning that, depending on the mode of operation, the cooling air 302 may comprise or consist of ambient air 306 from outside the data centre building. The adjustable ambient air inlet 305 can be operated to vary the amount of ambient air admitted into mixing chamber 307, in which it may be mixed with warm air 308 returned from the data centre (see Figure 9). Flow of warm air 308 into the mixing chamber 307 is controlled by operation of adjustable recycle vent 310 (see Figure 9). It will be understood that warm air 308 returned from the data centre is recycled cooling air 302 that has been used to air-cool items of electronic equipment in the data centre. The extent to which warm air 308 is mixed with ambient air 306 may be determined in dependence on the condition of the ambient air. Warm air 308 not returned to the mixing chamber may instead be exhausted from the data centre building, e.g. through exhaust vent 312, which exhaust vent 312 is operable to control the amount of warm air 308 so exhausted (see Figure 9). The cooling unit 301 comprises a sequence of items of air treatment / processing equipment, including a filter bank 317, mechanical cooling equipment 318 (including evaporator coils of a direct-expansion cooling system), wetted matrix adiabatic cooling system 319, and fan bank 320. The mechanical cooling equipment 318 forms parts of a mechanical cooling system also including a pump, a compressor and condensation coils not shown in Figure 8 (condensation coils are typically positioned outside the data centre building, such as on the roof). Between the wetted matrix adiabatic cooling system 319 and the fan bank 320, there is provided a heat exchanger space 350 sized and configured to accommodate a liquid-to-air heat exchanger, such as a set of cooling coils configurable to receive from the data centre warm liquid coolant and provide to the data centre cooled liquid coolant. Fans of the fan bank 320 are operable to draw air through the cooling unit 301, pushing air into exit chamber 351 and from there into the data centre (e.g. via a data centre air supply corridor, not shown in Figure 8) via supply air outlet 353. Cooling air provided by the cooling unit 301 may be referred to as 'supply air' 302, and may be directed into the data centre for use in air-cooling of electronic equipment in the data centre. For example, such supply air 302 may be transported along an air supply corridor to cold aisles for contact with air-cooled servers positioned between cold aisles and hot aisles (not shown in Figure 8). Such supply air may cool electronic equipment as it passes through the electronic equipment, thereby becoming warm air 308. Warm air may, for example be returned from hot aisles to an air return corridor or a warm air plenum, and from there be exhausted out of the data centre building and / or returned to the mixing chamber 307, as described above. Figure 9 shows a cut-away side, schematic view o the cooling unit 301 of Figure 8. As shown in Figure 9, the cooling unit 301 comprises a warm air return plenum 355 for transporting warm return air 308 from the data centre over the top of air treatment equipment to the mixing chamber 307 and / or to exhaust vent 312 for exhausting out of the data centre building. Although in the embodiment shown in Figure 9, the exhaust vent 312 is positioned on the top of the warm air return plenum 312, other configurations are envisaged, for example with the exhaust vent 312 positioned on a side of the warm air return plenum 312. Warm return air 309 enters the warm air return plenum 355 via return air inlet 354, which may for example be in fluid communication with data centre hot aisles, e.g. via a warm air return corridor and / or data centre warm air plenum (not shown in Figure 9). The cooling unit 301 further comprises recycle opening 352 between the exit chamber 351 and the warm air return plenum 355. In the configuration shown in Figures 8 and 9, supply air outlet 353 and return air inlet 354 are open (thus allowing cooling air 302 to be supplied to the data centre and warm return air 308 to be received from the data centre), and recycle opening 352 is closed (thus preventing cooling air 302 from passing directly into the warm air return plenum 355 from the exit chamber 351). Figure 10 shows another cut-away side, schematic view of the cooling unit 301 of Figures 8 and 9, reconfigured for liquid cooling. As shown in Figure 10, a liquid cooling unit is installed in heat exchanger space 350 between fan bank 320 and wetted matrix adiabatic cooling unit 319. The liquid cooling unit 321 comprises a liquid-to-air heat exchanger connected to flow and return pipework, allowing cooled cooling liquid 303 to be supplied to the data centre and warmed cooling liquid 304 to be received from the data centre for cooling. Supply air outlet 353 and return air inlet 354 are closed, and recycle opening 352 is open. During operation, air treated by the filter bank 317, mechanical cooling equipment 318 and wetted matrix adiabatic cooling system 319 is used to cool warm liquid coolant 304 by operation of the liquid cooling unit 321 thereby generating waste warm air 309. Waste warm air 309 is transported into the exit chamber 351 by fans of the fan bank 320, and from there passes through the recycle opening 352 into the warm air return plenum 355 (which now acts as a waste air recycle pathway). Waste warm air may be exhausted from the data centre building through exhaust vent 312, or recycled to the mixing chamber 307 via recycle pathway 124 and recycle vent 310. In the embodiment shown in Figure 10, the cooling unit 301 is fully converted to liquid cooling, meaning that the cooling unit supplies only cooled liquid coolant, and not cooling air, to the data centre. Conversion is achieved by inserting liquid cooling equipment in the heat exchanger space 350, and redirecting airflow by closing supply outlet 353 and return inlet 354, and opening recycle opening 352. However, it will be understood that conversion may instead be partial, thereby providing a hybrid cooling unit (such as a hybrid cooling unit similar to or substantially the same as that of the first embodiment of the invention). In that case, at least a portion of the cooling unit may be subdivided into an air cooling section and a liquid cooling section. Following that subdivision, liquid cooling equipment may be added only to the heat exchanger space of the liquid cooling section, the supply air outlet and return air inlet closed only in the liquid cooling section, and the recycle opening opened only in the liquid cooling section. Following such a conversion, the heat exchanger space 350 may remain unoccupied by liquid cooling equipment in the air cooling section, the supply air outlet and return air inlet may remain open in the liquid cooling section, and the recycle opening may remain closed in the liquid cooling section. Optionally, air filters and / or mechanical cooling equipment may be removed from the liquid cooling section. It will be understood that various variations of the embodiments shown in Figures 8-10 are envisaged. For example, mechanical cooling equipment may be omitted, and other types of adiabatic cooling system may be used, such as a spray system. Additionally or alternatively, fans may not be present prior to reconfiguration (in that case, fans may be added, at least to the liquid cooling section, during reconfiguration), for example if individual equipment fans are used to transport cooling air in the data centre. Furthermore, the order and / or positioning of equipment may be varied, provided that the adiabatic cooling unit (and mechanical cooling unit, if present) is positioned between the ambient air inlet and the heat exchanger space. Figure 11 shows steps of a method X00 according to a fourth embodiment of the invention. The method is for providing or increasing liquid cooling in a data centre, the data centre comprising electronic equipment requiring cooling, and cooling apparatus configured to supply cooling air to the electronic equipment. The cooling apparatus comprises an ambient air inlet for receiving ambient air from outside the data centre building, and at least one adiabatic cooling unit for cooling said ambient air to produce cooling air. The adiabatic cooling unit may be an evaporative cooling unit, such as a wetted matrix cooling unit, for example. The method comprises a first step X01 of providing data centre equipment configured to receive cool liquid coolant and produce warm liquid coolant. In step X02, at least one liquid-to-air heat exchanger is provided (e.g. installed in a cooling unit containing the cooling apparatus). In step X03, the data centre is configured to direct to the liquid-to-air heat exchanger at least a portion of the cooling air and at least a portion of said warm liquid coolant so that the liquid-to-air heat exchanger is operable to exchange heat from the warm liquid coolant to said portion of the cooling air and thereby provide warm exhaust air and cool liquid coolant. It will be understood that step X03 may comprise providing in the data centre liquid coolant pipework for transporting liquid coolant to and from the electronic equipment requiring cooling. In step X04, the data centre is configured to direct at least a portion of the warm exhaust air out of the data centre. In step X05, the data centre is configured to direct at least a portion of the warm exhaust air to an air mixing chamber arranged to receive and mix (a) ambient air received via the ambient air inlet and (b) at least a portion of said warm exhaust air. The air mixing chamber is located between the ambient air inlet and the adiabatic cooling unit. Prior to performance of the method, the cooling apparatus may be configured for direct air cooling or indirect air cooling. When configured for indirect air cooling, the cooling apparatus may comprise at least one air-to-air heat exchanger configured to receive warm return air from the data centre and a first portion of the cooling air, and exchange heat from the warm return air to the first portion of the cooling air, thereby providing cooled supply air for the data centre and first warm exhaust air. When the cooling apparatus is configured for indirect air cooling, step X01 may additionally include removing from the cooling apparatus at least one said an air-to-air heat exchanger and replacing it with at least one said liquid-to-air heat exchanger. In certain embodiments, the method may be performed to provide a cooling unit (such as a hybrid cooling unit) according to the first or second embodiment of the invention. Alternatively, the method may be performed to effect complete conversion of the cooling apparatus so that the cooling apparatus does not supply cooling air to the data centre and instead provides only liquid coolant. It will be understood that the method may be performed to convert a cooling unit from that shown in Figures 8 and 9 to that shown in Figure 10. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, 5 advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A cooling unit for a data centre in a data centre building, wherein the cooling unit is configured to provide the data centre with cooled liquid coolant, wherein the cooling unit comprises:(i) an ambient air inlet for receiving ambient air from outside the data centre building;(ii) at least one adiabatic cooling unit for cooling ambient air to produce cooling air;(iii) at least one liquid-to-air heat exchanger configured to: receive warm liquid coolant from the data centre and at least a portion of the cooling air, and exchange heat from the warm liquid coolant to said portion of the cooling air, thereby providing cooled liquid coolant for the data centre and warm exhaust air;(iv) a warm air outlet for expelling at least a portion of the warm exhaust air, wherein the warm air outlet is in fluid communication with air outside the data centre building; and(v) an air mixing chamber configured to receive:ambient air received via the ambient air inlet, and one or more of:at least a portion of the warm exhaust air, and warm air from the data centre;wherein the air mixing chamber is located between the ambient air inlet and the adiabatic cooling unit so that ambient air passes through the mixing chamber prior to contact with the adiabatic cooling unit.

2. The cooling unit of claim 1, comprising a plurality of fans operable to transport air from the ambient air inlet to the warm air outlet.

3. The cooling unit of claim 1 or claim 2, comprising a recycle airflow conduit configured to transport said warm exhaust air to the warm air outlet and to the air mixing chamber.

4. The cooling unit of any preceding claim, wherein the adiabatic cooling unit is an evaporative cooling unit, such as a wetted matrix cooling unit.

5. The cooling unit of any preceding claim, wherein the liquid-to-air heat exchanger is arranged for contact with the cooling air and configured to receive warm liquid coolant, and optionally one or more pumps for circulating liquid coolant through the at least one liquid-to-air heat exchanger.

6. The cooling unit of any preceding claim, wherein the cooling unit is a hybrid cooling unit configured additionally to provide the data centre with supply air suitable for cooling electronic equipment in the data centre.

7. The hybrid cooling unit of claim 6, wherein the hybrid cooling unit is a direct air hybrid cooling unit configured to provide supply air for the data centre comprising said ambient air.

8. The hybrid cooling unit of claim 7, configured to provide a first portion of cooling air to the liquid-to-air heat exchanger, and to provide a second portion of cooling air as the supply air for the data centre, wherein the first portion of cooling air is segregated from the second portion of cooling air by an air barrier so that the second portion of cooling air does not come into contact with the liquid-to-air heat exchanger and the first portion of cooling air is not used as supply air.

9. The hybrid cooling unit of claim 7 or claim 8, wherein the air mixing chamber is configured to receive:said ambient air received via the ambient air inlet, at least a portion of said warm exhaust air, andat least a portion of warm return air from the data centre; optionally wherein the hybrid cooling unit additionally comprises an adjustable return inlet operable to control the amount of warm return air admitted into the mixing chamber, and / or an adjustable recycle inlet to control the amount of warm exhaust air admitted into the mixing chamber.

10. A hybrid cooling unit according to claim 6, wherein the hybrid cooling unit is an indirect air hybrid cooling unit comprising at least one air-to-air heat exchanger configured to:receive warm return air from the data centre and a first portion of cooling air, andexchange heat from the warm return air to said first portion of the cooling air, thereby providing supply air for the data centre and additional warm exhaust air.

11. The hybrid cooling unit of claim 10, configured to provide a first portion of cooling air to the liquid-to-air heat exchanger, and to provide a second portion of cooling air to the air-to-air heat exchanger, wherein the first portion of cooling air is segregated from the second portion of cooling air by an air barrier so that the second portion of cooling air does not come into contact with the liquid-to-air heat exchanger and the first portion of cooling air is not contacted with the air-to-air heat exchanger.

12. The hybrid cooling unit of claim 10 or claim 11, comprising an internal airflow path segregated from an external airflow path,wherein the internal airflow path extends from a return air inlet to a supply air outlet, the return air inlet being configured to receive warm return air from the data centre;and wherein the external airflow path extends from the ambient air inlet to the warm air outlet;optionally wherein the hybrid cooling unit comprises a plurality of fans for transporting air along the internal airflow path.

13. The hybrid cooling unit of any one of claims 6 to 12, comprising a supply air outlet arranged to convey the supply air to the data centre, optionally wherein the supply air outlet is in fluid communication with one or more cold zones of the data centre, such as one or more cold corridors or cold aisles.

14. The hybrid cooling unit of any one of claims 6 to 13, wherein the hybrid cooling unit is sub-divided by an air barrier into an air cooling section and a liquid cooling section, wherein the air cooling section is operable to provide the supply air and the liquid cooling section is operable to provide the cooled liquid coolant, wherein the liquid cooling section and at least a portion of the air cooling section is in fluid communication with the mixing chamber, and wherein the air cooling section and the liquid cooling section each comprise an independently controllable adiabatic cooler operable to treat air received from the mixing chamber.

15. The hybrid cooling unit of claim 14, wherein the air cooling section and the liquid cooling section each comprise one or more independently controllable fans.

16. The hybrid cooling unit of claim 14 or claim 15, wherein the air cooling section has a maximum ambient airflow rate and the liquid cooling section has a maximum ambient airflow rate, and wherein the maximum ambient airflow rate of the liquid cooling section is two to six times the maximum ambient airflow rate of the air cooling section.

17. The hybrid cooling unit of any one of claims 6 to 16, configured to provide one or more of:500 kW to 1MW of combined air and liquid cooling capacity;75 to 150 kW air cooling capacity;425 to 850 kW of liquid cooling capacity;cooling air at a maximum rate of 40 to 60 m3 / s;supply air at a temperature of from 18°C to 32°C and a relative humidity of from 20% to 80%; and / orcooled liquid coolant at a temperature of from 25°C to 45°C, such as 30°C to 40°C, optionally wherein the hybrid cooling unit is configured to operate with a warm coolant return temperature of from 35°C to 50°C, such as 40°C to 50°C, and / or optionally wherein the hybrid cooling unit is configured to cool liquid coolant by 5°C to 15°C, such as 7.5°C to 12.5°C.

18. A method of providing or increasing supply of liquid cooling fluid in a data centre, wherein the data centre comprises cooling apparatus configured to provide supply air for air-cooling electronic equipment in the data centre, and wherein the cooling apparatus comprises an adiabatic cooling unit operable to treat ambient air from outside the data centre to produce cooling air; wherein the method comprises:(i) providing at least one liquid-to-air heat exchanger, and configuring the liquid-to-air heat exchanger to receive warm liquid coolant from items of electronic equipment in the data centre;(ii) configuring the data centre to direct to the liquid-to-air heat exchanger at least a portion of the cooling air and at least a portion of said warm liquid coolant so that the liquid-to-air heat exchanger is operable to exchange heat from the warm liquid coolant to said portion of the cooling air and thereby provide warm exhaust air and cool liquid coolant;(iii) configuring the cooling apparatus so that:at least a portion of the warm exhaust air can be exhausted out of the data centre, andambient air in the mixing chamber can be combined with at least one of:at least a portion of the warm exhaust air, and warm air from the data centre, wherein the mixing chamber is located between an ambient air inlet and the adiabatic cooling apparatus so that the adiabatic cooling apparatus receives ambient air via the mixing chamber.

19. The method of claim 18, wherein the cooling apparatus comprises at least one air-to-air heat exchanger configured to:receive warm return air from the data centre and at least a portion of the cooling air, andexchange heat from the warm return air to said at least a portion of the cooling air, thereby providing cooled supply air for the data centre and further warm exhaust air;wherein the method comprises removing from the cooling apparatus at least one said air-to-air heat exchanger and replacing it with at least one said liquid-to-air heat exchanger.

20. The method of claim 18 or claim 19, comprising providing in the data centre liquid coolant pipework for transporting liquid coolant to and from electronic equipment requiring liquid cooling.

21. The method of any one of claims 18 to 20, wherein the method provides a hybrid cooling unit according to any one of claims 6 to 17.

22. The method of any one of claims 18 to 20, wherein the method comprises complete conversion of the cooling unit so that the cooling unit does not supply cooling air to the data centre and instead provides only liquid coolant.

23. An air handling unit for a data centre building, wherein the air handling unit comprises:(i) an ambient air inlet for receiving ambient air from outside the data centre building;(ii) at least one adiabatic cooling unit for cooling ambient air to produce cooling air, optionally an evaporative cooling unit such as a wetted matrix cooling unit;(iii) a cooling air outlet for conveying the cooling air to the data centre;(iv) optionally, a plurality of fans for transporting air from the ambient air inlet to the cooling air outlet;(v) a heat exchanger space suitable for accommodating at least one liquid-to-air heat exchanger, wherein the space is positioned between the adiabatic cooling unit and the cooling air outlet;(vi) a warm air outlet for expelling out of the data centre building at least a portion of warm exhaust air from the heat exchanger space; and(vii) an air mixing chamber for receiving ambient air via the ambient air inlet and at least one of:at least a portion of warm exhaust air from the heat exchanger space, andwarm air from the data centre, wherein the air mixing chamber is located between the ambient air inlet and the adiabatic cooling unit;wherein the warm air outlet is closed and the cooling air outlet is open.

24. An air handling unit according to claim 23, comprising a recycle conduit extending from an exit chamber downstream of the adiabatic cooling unit to the mixing chamber.

25. An air handling unit according to claim 24, wherein the air handling unitcomprises a first recycle opening between the exit chamber and the recycleconduit, and a second recycle opening between the recycle conduit and the mixing chamber, and wherein one or both of the first and second recycle openings is closed.

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