Ventilation system

Abstract

A ventilation system delivers air to the interior of a housing and comprises a heat exchanger 100 recovering heat by heat exchange between an outgoing heated fluid and an incoming supply air stream. A

Description

The invention relates to a ventilation system for delivering air to the interior of a housing. In particular the invention relates to ventilation systems incorporating a heat exchanger configured to recover heat by heat exchange between an outgoing stream of heated fluid and an incoming stream of supply air. Background Heat recovery systems are often incorporated into ventilation systems in order to recover waste heat by transferring the heat from heated exhaust fluid into a cooler incoming stream of supply fluid. In many fresh-air ventilation systems, for example, heat may be exchanged between heated exhaust air and cooler incoming fresh air, to raise the temperature of the incoming supply air before it reaches the interior of a housing. This heat recovery helps to reduce heating costs where it is important to maintain the interior of a housing above a threshold temperature. In applications such as livestock housing, it is important to maintain the interior of a housing within a suitable temperature range so that the livestock are kept in optimum conditions. While heat exchangers have been incorporated into the ventilation systems which provide fresh air to livestock sheds, these systems are typically manually operated and controllable only with ON / OFF controls, so that there is no way of controlling the proportion of heat being recovered. Separate gas-fired heaters are typically positioned inside the housing so that additional heat can be added to the housing by turning on the heaters when the inside temperature gets too low. Such systems are typically cumbersome and inefficient, and the reliance on manual control of ventilation and heating increases the risk that the conditions in the housing can fall outside desired ranges and become either too hot or too cold. This has a knock-on effect on the welfare of livestock inside the housing, as well as on the costs of system operation. Even the most advanced existing systems do not fully optimise the operation of the system taking into account thermodynamic, economic and environmental variables. Summary of Invention The invention provides a livestock-housing ventilation system and a livestock housing comprising a ventilation system, as defined in the appended independent claims to which reference should now be made. Preferred or advantageous features of the invention are set out in the dependent subclaims. According to a first aspect of the invention there is provided a ventilation system for delivering air to the interior of a housing. The ventilation system comprises: a heat exchanger configured to recover heat by heat exchange between an outgoing stream of heated fluid and an incoming stream of supply air, and a variable-speed supply fan configured to draw the stream of supply air through the heat exchanger and into the housing. The system further comprises an inside temperature sensor configured to sense a temperature of the air inside the housing, a supply air temperature sensor configured to sense a temperature of the supply air. The supply air temperature sensor is preferably configured to sense a temperature of the supply air before it passes through the heat exchanger. The ventilation system comprises a controller which is configured to receive temperature data from the temperature sensors and to control the supply fan. The controller is configured to calculate an input heat requirement of the housing based on the temperature data received from the temperature sensors. The controller is preferably programmed to operate in an energy control mode in which the controller sets the speed of the variable-speed supply fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing. In a preferred embodiment, the system additionally comprises a variable speed exhaust fan which is configured to remove air from the interior of the housing and to direct the extracted air through the heat exchanger before exhausting it to the atmosphere. In such embodiments, the controller is preferably programmed to operate in an energy control mode in which the controller sets the speed of the variable-speed supply fan and the variable-speed exhaust fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing. The energy control mode of present ventilation system provides the significant benefit of meeting the input heat requirement, and thus maintaining the interior of the housing at the required temperature, and minimising electricity consumption. In addition to or alternatively to the energy control mode, the controller of the first aspect may be programmed to operate in any of the cost-saving mode, carbon-saving mode, or energy recovery maximise modes described below. The use of a variable-speed supply fan enables the controller to adapt the energy consumption of the ventilation system more precisely and efficiently than is possible in prior art systems which employ single-speed fans which can only be off or on (running at full speed). The energy consumed to drive an electric fan goes up in proportion to the cube of the fan speed, so the present inventor has appreciated that controlling the system to operate at the minimum-acceptable fan speed can yield significant energy savings compared to straightforward conventional systems without intelligent fan speed control. As described below, the present ventilation system may be programmed to operate in a variety of different modes to enable holistic control of the climactic conditions inside the housing. In preferred embodiments, the ventilation system may be operated differently depending on the control mode selected by a user, so that the controller optimises the operating settings in the way preferred by the user. When operating in this energy control mode, the electricity consumption is advantageously automatically maintained at the lowest level possible without compromising on the climactic conditions in the interior of the housing. The controller may advantageously be configured to continuously monitor the inside temperature and to automatically vary the fan speed as required so that the stream of supply air provides the appropriate heat input to the interior of the housing, for the lowest possible electricity input. In addition to the variable-speed supply fan, the system may include an electricity-powered variable-speed exhaust fan and / or an electric heater for directly heating the inside of the housing. The system may be operated to provide the input heat requirement by recovering heat from the heat exchanger, and / or by direct heating using the heater, but as these options will have different efficiencies, the power-consumption implications of operating the systems in different ways may be very different. For example, if the controller has the option of heating the interior of the housing using heat recovery driven by the fans in the heat exchanger, an electric heater or an electric heat-pump, the most electricity-efficient option may depend on the weather conditions and the heat-differential usable by the heat pump. The controller may advantageously be programmed to account for the efficiencies of the available heat sources in order to calculate the system settings which will provide the required input heat at the overall lowest electricity consumption. The controller is preferably programmed to operate in an energy control mode in which the controller operates any electrical heaters and sets the speed of the variable-speed fan speeds at levels which minimise the electricity consumed by the system while meeting the input heat requirement of the housing. The controller is preferably configured to calculate the input heat requirement by comparing the inside temperature sensed by the inside temperature sensor with a target temperature for the interior of the housing. Depending on the sensed inside temperature and the target temperature of the interior of the housing, the input heat requirement may be positive (if heating is required to increase the inside temperature) or negative (if cooling is required to decrease the inside temperature). The controller is preferably configured to calculate a recovered heat value for the heat transferred to the incoming stream of supply air in the heat exchanger. The controller may be configured to control the recovered heat value by controlling the speed of the supply fan to determine a flow rate of the stream of supply air through the heat exchanger. The controller may also be configured to control the recovered heat value by controlling a rate at which the outgoing stream of heated fluid passes through the heat exchanger. By controlling the residency times of the incoming supply air and / or the outgoing heated fluid, the rate of heat transfer from the outgoing to incoming pathways of the heat exchanger may be varied, so that the recovered heat transferred into the supply air is varied. In the event that the inside of the housing is too hot and the input heat requirement is negative, the controller may be configured to prevent heat recovery by directing heated exhaust air out of a housing outlet instead of through the heat exchanger. The controller may be configured to calculate a supply air heat value which quantifies the heat energy provided to the interior of the housing by the stream of supply air. The controller may calculate the supply air heat value by combining the sensed supply air temperature (the temperature of the supply air before heat exchange) with the recovered heat value. To calculate the speed of the variable-speed supply fan which minimises the electricity consumed by the system while meeting the input heat requirement of the housing, the controller may be programmed to compare the supply air heat value with the input heat requirement of the housing, and to calculate a minimum supply fan speed at which the supply air heat value will match the input heat requirement of the housing.As described further below, the present ventilation system may be used in a variety of applications in order to maximise the efficiency of the climate control in the interior of a housing. For example the ventilation system may be applied to a liquid-to-air heat exchanger in a data centre, in which fresh air ventilation and liquid cooling are used to control the temperature of IT equipment. Alternatively the ventilation system may be a livestock housing ventilation system with air-to-air heat exchange, in which livestock in the housing must be kept at an appropriate target temperature which may require warming (for example in winter) or cooling (for example in summer). Liquid-Air Heat Exchange In one preferred embodiment, the outgoing stream of heated fluid may be an outgoing stream of heated liquid coolant. The interior of the housing may contain IT equipment, and the outgoing stream of heated liquid coolant may be heated by heat generated by the IT equipment during operation. As the IT equipment generates heat during use, the interior of the housing typically requires cooling, so the input heat requirement is typically negative. In this embodiment, the incoming stream of supply air may be cool air which is delivered to the interior of the housing to directly cool the IT equipment, or the incoming stream of supply air may be used simply to absorb heat from the liquid coolant in the heat exchanger, so that the liquid coolant can be cooled in the heat exchanger and recirculated to provide further liquid cooling to the IT equipment in the housing. Air-Air Heat Exchange In an alternative preferred embodiment, the outgoing stream of heated fluid is an outgoing stream of heated exhaust air. In such embodiments, the ventilation system preferably comprises an exhaust fan configured to direct the stream of exhaust air out of the housing through the heat exchanger, and the controller is configured to control the exhaust fan. Particularly preferably the exhaust fan may be a variable-speed fan, and the controller is configured to individually control the speed of both the supply fan and the exhaust fan. In this embodiment, the controller may be programmed to operate in an energy control mode in which the controller sets the speed of the variable-speed supply fan and the variablespeed exhaust fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing. The controller may be configured to control the recovered heat value by controlling the speed of the supply fan to determine a flow rate of the stream of supply air through the heat exchanger, and / or by controlling the speed of the exhaust fan to determine a flow rate of the stream of exhaust air through the heat exchanger. In applications which may require additional heating as well as recovered heat to meet the input heat requirement of the housing, the system may comprise a heater configured to heat air inside the housing. The controller may then be configured to receive temperature data from the temperature sensors, and to control the supply fan, the exhaust fan and the heater. The heater may be, for example, a gas-fired heater, an electric heater, or an electric heatpump. In some embodiments, the system may have multiple heaters which use alternative fuels - for example a gas-fired heater and an electric heat-pump - in order to provide multiple options for heating the interior of the housing with different financial and carbon costs. If the heater has only ON or OFF settings, then the controller may control the energy consumption of the heater by controlling the ON time of the heater. If the heater has variable temperature settings or thermostatic control, then the controller may control the ON time and the heat output of the heater to control the energy consumption of the heater. In order to operate the ventilation system in energy control mode, the controller may advantageously be configured to monitor the energy consumption of the heater as well as the energy consumption of the variable speed fan(s). The controller may preferably be programmed to take into account the recovered heat value, the fan electricity consumption and the heater energy consumption in order to operate the system in energy control mode. The controller may be configured to calculate the input heat requirement of the housing by comparing the inside temperature sensed by the inside temperature sensor with a target temperature for the interior of the housing, and to calculate the required input heat from the heater and / or the recovered heat value provided by the supply air which has passed through the heat exchanger. In the energy control mode, the controller is preferably configured to calculate the total energy consumed by the variable-speed fan(s) and the heater, and to set the speed of the variable-speed fan(s) and the on-time or heat output of the heater at levels which minimise the total energy consumed by the system while meeting the input heat requirement of the housing. In energy control mode the controller may preferably be programmed to run the variablespeed fan at a speed of between 55% and 75% of its maximum speed, or between 60% and 70% of its maximum speed. As shown in the Figures, this is typically the most energyefficient running speed for a ventilation system in which heat recovery is driven by a variable speed fan, as fan speeds in this range maximise the balance of recovered-heat benefit with fan energy consumption. The controller is preferably programmed to operate the system in one of a plurality of control modes. The energy control mode described above is preferably one of the plurality of control modes. The controller may be programmed to preferentially supply recovered heat or to preferentially supply heat from the heater, depending on the control mode selected. The controller may be configured to operate in one of the plurality of control modes at a time in response to a user input indicating a user priority. For example if a user has the priority of minimising electricity consumption, then the user may set the controller to operate the system in energy control mode. If the user would prefer to prioritise a different factor, the user may instead set the controller to operate in one of the alternative control modes described below. In order to enable the operation of the system in different control modes, the controller may be configured to receive live data on system electricity consumption and gas consumption and on any other fuel or power consumed by the system. The controller may be configured to receive live data on the cost of the electricity and / or gas and / or any other fuel or power supplied to the system. The controller may be programmed to assign carbon values to electricity and / or gas and / or any other fuel or power consumed by the system. Cost-Saving Mode The plurality of control modes may optionally comprise a cost-saving mode. In cost-saving mode, the controller may calculate the economic cost of electricity and optionally gas consumed by the variable-speed fan(s) and the heater. The controller may then set the speed of the variable-speed fan(s) and an on-time or heat output of the heater at levels which minimise the total cost of the electricity and gas (if used) consumed by the system while meeting the input heat requirement of the housing. In cost-saving mode, the controller may be programmed to calculate the fan speeds and the heater settings required to meet the input heat requirement at the lowest economic cost, and to run the fans and the heater at those operating conditions. The cost of electricity may be subject to variable tariffs based on time of day or other more sophisticated contracts. The controller is preferably programmed with an optimisation algorithm, and the controller is preferably programmed to take any variations into electricity cost into account when determining the optimum economic operating point. Particularly advantageously, the controller may be programmed to take into account the differing prices of different forms of input energy, and to vary the operation of the ventilation system to minimise the overall operating cost. For example various embodiments of the ventilation system may be powered by electricity from the mains grid, electricity from renewable sources, fossil-fuel-powered generators, bio-fuels or gas from the mains. The controller may advantageously be programmed to take into account the different economic costs of these different fuels, and to operate the system using the cheapest overall combination of fuels which still allow the system to meet the input heat requirement of the housing. In order to provide the required heat to the housing the controller may for example choose between recovered heat from the heat exchanger, which can be controlled by varying supply and / or exhaust fan speeds with the ensuing implications in electricity cost, or heat from the heater. The choice of which of these options to use in turn creates a financial cost to pay for the fuel powering the ventilation system. The present system may advantageously be operable at the lowest possible financial cost by monitoring fluctuations in the prices of alternative fuels, and operating the fans and / or heater at levels which will provide the required input heat at the lowest financial cost. As different heating options may have different efficiencies, the financial implications of operating the systems at different settings may be very different. For example, if the controller has the option of heating the interior of the housing using a gas heater or an electric heat-pump, the most efficient option may depend on the weather conditions and the heat-differential usable by the heat pump. The controller may advantageously be programmed to account for the efficiencies of the available heat sources in order to calculate the system settings which will provide the required input heat at the overall lowest cost. In a preferred embodiment, the heater is a gas heater and the fans are electric fans. In cost-saving mode the controller may be programmed to calculate the financial cost of providing the input heat requirement from the gas heater, or from heat recovered in the heat exchanger, or from a combination of the two, and to operate the system at the settings with the lowest cost. In cost-saving mode the controller may be programmed to run the variable-speed fan at a speed of between 40% and 65% of its maximum speed, or between 50% and 60% of its maximum speed. As shown in the Figures, this is typically the most cost-efficient running speed for a ventilation system in which heat recovery is driven by a variable speed fan and direct heating is provided by gas, though this will be highly dependent on fluctuating utility prices. In order to combat this, the controller is preferably programmed to receive live data on utility prices so that the system can be controlled in the most cost effective way at any given time. Carbon-Saving Mode The plurality of control modes may optionally comprise a carbon-saving mode, in which the controller calculates the carbon footprint of the electricity and optionally gas consumed by the variable-speed fan and the heater, and in which the controller sets the speed of the variable-speed fan and the on-time or heat output of the heater at levels which minimise the carbon footprint of the electricity and gas (if used) consumed by the system while meeting the input heat requirement of the housing. In carbon-saving mode, the controller may be programmed to calculate the fan speeds and the heater on-time or heat output required to meet the input heat requirement at the lowest carbon footprint, and to run the fans and the heater at those operating conditions. Similarly to the variability in fuel prices, the carbon footprint of the ventilation system may be significantly affected by which fuel sources the controller uses to provide a heat input. For example the controller may be able to increase heat input to the required level either by controlling the fan speeds to increase the recovered heat value (which may result in an increase in electricity consumption) or by increasing the heat output from the heater. If the heater is powered by gas, then the carbon footprint from operating the gas-powered heater will be different from that created by increasing the recovered heat value. In some cases, energy sources with lower carbon footprints may have a higher financial cost. The preferred system settings in carbon-saving mode may thus be very different from the settings preferred in cost-saving mode or energy-saving mode. In carbon-saving mode the controller may advantageously prioritise the carbon-footprint of the ventilation system, and operate the fans and / or heater in a way that meets the input heat requirement and minimises the carbon footprint of the system, even if this increases the financial running cost of the system. This may be particularly suitable for users who wish to prioritise the lowering of their carbon footprint regardless of the implications in running cost. In a preferred embodiment, the heater is a gas heater and the fans are electric fans. In carbon-saving mode the controller may be programmed to calculate the carbon footprint of providing the input heat requirement from the gas heater, or from heat recovered in the heat exchanger, or from a combination of the two, and to operate the system at the settings with the lowest carbon footprint. In carbon-saving mode the controller may be programmed to run the variable-speed fan at a speed of between 30% and 50% of its maximum speed, or between 35% and 45% of its maximum speed. As shown in the Figures, in certain embodiments this is the most costefficient running speed for a ventilation system in which heat recovery is driven by a variable speed fan and direct heating is provided by gas, though this will be highly dependent on whether power is provided from renewable sources. In order to combat this, the controller is preferably programmed to receive live data on the source and the resulting carbon footprint of the power being consumed, so that the system can be controlled in the most carbon-efficient way at any given time. Energy-Recovery Maximise Mode The plurality of control modes may comprise an energy recovery maximise mode, in which the controller sets the speeds of the supply fan and the exhaust fan at levels which maximise the recovered heat value for the heat transferred to the incoming stream of supply air in the heat exchanger. Heat Exchanger The heat exchanger may contain an incoming air pathway arranged to receive the stream of supply air from an air inlet, and an exhaust air pathway arranged to direct a stream of exhaust air from the housing to an exhaust. The heat exchanger may be configured so that heat is transferred from the exhaust air pathway to the incoming air pathway to heat the stream of supply air to a supply temperature before it reaches the interior of the housing. The controller is preferably configured to control the recovered heat value and the temperature of the supply air after it has passed through the heat exchanger by controlling the speed of the supply fan to determine a flow rate of the stream of supply air through the incoming air pathway, and / or by controlling the speed of the exhaust fan to determine a flow rate of the stream of exhaust air through the exhaust air pathway. Alternative Aspect In a second aspect, there is provided a ventilation system for delivering air to the interior of a housing, the system comprising: a heat exchanger configured to recover heat by heat exchange between an outgoing stream of heated exhaust air and an incoming stream of supply air; a supply fan configured to draw the stream of supply air through the heat exchanger and into the housing; an exhaust fan configured to direct the stream of exhaust air out of the housing through the heat exchanger; an inside temperature sensor configured to sense a temperature of the air inside the housing; a supply air temperature sensor configured to sense a temperature of the supply air; and a controller configured to receive temperature data from the temperature sensors, and to control the supply fan and the exhaust fan; in which at least one of the supply fan or the exhaust fan is a variable-speed fan; in which the controller is configured to calculate an input heat requirement of the housing based on the temperature data received from the temperature sensors. Preferably the supply fan is a variable-speed supply fan and the exhaust fan is a variablespeed exhaust fan. The controller is preferably programmed to operate the system in an energy control mode, or a cost-saving mode, or a carbon-saving mode, or an energy-recovery-maximise mode, as described above. The controller may be programmed to operate in an energy control mode which sets the speed of the variable-speed fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing. In addition to or alternatively to the energy control mode, the controller of the first aspect may be programmed to operate in any of the cost-saving mode, carbon-saving mode, or energy recovery maximise modes described above. The ventilation system of the second aspect preferably incorporates the features of the first aspect in a system which includes an air-to-air heat exchanger. Any of the features described above may be applied to the ventilation system of the second aspect. In some embodiments, the supply fan is a variable-speed fan, and in which the controller is programmed to operate in an energy control mode which sets the speed of the variablespeed supply fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing. By varying the speed of the supply fan, the controller may vary the electricity consumption of the ventilation system, and also control the recovered heat value by controlling how much heat the incoming stream of supply air absorbs in the heat exchanger. Alternatively the exhaust fan may be a variable-speed fan and the supply fan may be a single-speed fan. In this embodiment the controller may operate in an energy control mode which sets the speed of the variable-speed exhaust fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing. By varying the speed of the exhaust fan, the controller may vary the electricity consumption of the ventilation system, and also control the recovered heat value by controlling how much heat the outgoing stream of exhaust air loses in the heat exchanger. Particularly preferably both the supply fan and the exhaust fan are variable-speed fans, and the controller is configured to individually control the speed of each fan. The controller may be configured to control the recovered heat value by controlling the speed of the supply fan to determine a flow rate of the stream of supply air through the heat exchanger, and / or by controlling the speed of the exhaust fan to determine a flow rate of the stream of exhaust air through the heat exchanger. The system may comprise a heater configured to heat air inside the housing, and the controller may be configured to receive temperature data from the temperature sensors, and to control the supply fan, the exhaust fan and the heater. The controller may be programmed to operate the ventilation system in any of the control modes described above in relation to the first aspect. The controller may be programmed to operate the ventilation system in a cost-saving mode, in which the controller calculates the economic cost of the electricity and / or gas consumed by the ventilation system. For example the controller calculates the economic cost of the electricity consumed by the electric variable-speed fan(s) and the electricity or gas consumed by the heater (if present). The controller then sets the speed of the variablespeed fan and an on-time or output level of the heater at levels which minimise the cost of the power (electricity and optionally gas if a gas-powered heater is used) consumed by the system while meeting the input heat requirement of the housing. The controller preferably receives data on the price of the various sources of power used by the ventilation system, including market prices and any applicable variable-rate utility tariffs. The controller then calculates the lowest-cost settings which will still provide the input heat requirement of the housing, and operates the ventilation system at those settings. The controller preferably recalculates and updates the operating settings at regular intervals to account for any price fluctuations and changes in the temperature of the supply air. The controller may be programmed to operate the ventilation system in a carbon-saving mode, in which the controller calculates the carbon footprint of the electricity and / or gas consumed by the ventilation system. For example the controller calculates the economic cost of the electricity consumed by the electric variable-speed fan(s) and the electricity or gas consumed by the heater (if present). The controller then sets the speed of the variablespeed fan(s) and the on-time or output of the heater at levels which minimise the carbon footprint of the electricity and gas consumed by the system while meeting the input heat requirement of the housing. The controller preferably receives data on the carbon footprint of the various sources of power used by the ventilation system, which may vary depending on whether the ventilation system is set up to receive renewable electricity. The controller may, for example, operate the ventilation system to preferentially meet the input heat requirement of the housing using renewable electricity to recover heat using the fans and add direct heat using any available electric heater, rather than meeting the input heat requirement by using gas to power a gas heater. The controller may be programmed to operate the ventilation system in an energy recovery maximise mode, in which the controller sets the speeds of the supply fan and the exhaust fan at levels which maximise the recovered heat value for the heat transferred to the incoming stream of supply air in the heat exchanger. Livestock Housing In either of the first or second aspects described above, the housing may be a livestock housing. The ventilation system may thus be a livestock-housing ventilation system for delivering air to the interior of a livestock housing for containing livestock. The livestock housing may be any building or structure used to house livestock, or domesticated animals. For example, the livestock housing may be a barn, shed or warehouse used to house for cattle, sheep, pigs or other farmed animals during at least part of the animals’ life cycles or during at least part of the year. The livestock housing may be a housing for fowl such as turkeys, ducks or chickens. For example, the livestock housing may be a shed used to house chickens as they are raised from hatching. Although it will be understood that the present invention is equally applicable to livestock housing for a wide range of livestock, in the following description the invention will be described by reference to chickens, and in particular broilers, which are chickens raised for meat. In many climate-control applications, for example cooling IT equipment in data centres, the target temperature and input heat requirement are predictable and likely to be varied only to compensate for ambient conditions that affect the temperature of incoming supply air. In livestock farming, however, the climactic requirements of the livestock housing vary depending on the type and age of the livestock, weather conditions, and a number of animal welfare factors which do not need to be taken into account in other industries. For these reasons, climate-control systems for livestock housings are typically basic and manually controlled, with cooling provided by fans driving fresh-air ventilation, and heating provided by gas-fired heaters inside the livestock housing. Heat-exchangers are employed in some systems to improve efficiency, but the operation of existing livestock housing ventilation systems is typically inefficient and costly. Huge numbers of livestock are raised worldwide every year, with many varieties of livestock spending at least part of the year living in livestock-housing to shelter them from adverse weather. Some varieties of livestock, for example fowl such as chickens, ducks, or turkeys, may spend some or all of their lives inside livestock housing such as large purpose-built sheds. In a preferred embodiment, there is provided a livestock-housing ventilation system having the features described above in relation to the first or second aspect. In a third aspect, there is provided a livestock-housing ventilation system for delivering air to the interior of a livestock housing, the system comprising: a heat exchanger configured to recover heat by heat exchange between an outgoing stream of heated exhaust air and an incoming stream of supply air; a supply fan configured to draw the stream of supply air through the heat exchanger and into the housing; an exhaust fan configured to direct the stream of exhaust air out of the housing through the heat exchanger; an inside temperature sensor configured to sense a temperature of the air inside the housing; a supply air temperature sensor configured to sense a temperature of the supply air; and a controller configured to receive temperature data from the temperature sensors, and to control the supply fan and the exhaust fan; in which at least one of the supply fan or the exhaust fan is a variable-speed fan; in which the controller is configured to calculate an input heat requirement of the housing based on the temperature data received from the temperature sensors. The controller is preferably programmed to operate the system in an energy control mode, or a cost-saving mode, or a carbon-saving mode, or an energy-recovery-maximise mode, as described above. The controller may be configured to calculate the input heat requirement of the housing based on the temperature data received from the temperature sensors and a heat output of livestock in the livestock housing. As livestock metabolise food and generate body-heat, the controller may advantageously be programmed to account for this heat output as a factor when calculating the input heat requirement of the housing. The controller may advantageously be programmed to account for the varying level of metabolic heat generation from the livestock depending on their age and species, and to run the ventilation at the operating settings dictated by the user’s choice of energy- / cost- / carbon-saving / energy-recovery-maximising control mode. By controlling the ventilation system at the optimum settings for the user’s priorities, the controller may advantageously control the recovered heat value so that no external heating is required as early as possible in the residence time of a crop of livestock in the housing. This may advantageously allow high welfare standards to be maintained at the lowest possible environmental and / or economic cost. Description of Specific Embodiments of the Invention Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating a heat exchanger usable in ventilation systems according to the present invention; Figure 2A is a graph illustrating the operating considerations relating to a variable-speed fan in embodiments of the present invention; Figure 2B illustrates the economic optimum in the graph of Figure 2A; Figure 3 illustrates the energy consumption, economic cost and carbon cost of operating a variable-speed fan in embodiments of the present invention; Figure 4 is a schematic diagram illustrating system controls usable by a controller in a ventilation system according to an embodiment of the present invention; Figure 5 illustrates variables which are controllable by the controller in embodiments of the present invention; Figure 6A illustrates alternative factors which may be prioritised by a given user; Figure 6B illustrates the variables which may be prioritised to affect the factors of Figure 6A; Figure 7A is a schematic illustration of the heat input factors which affect the heat balance in a typical livestock housing; and Figure 7B is a schematic illustration showing how the heat balance of a livestock housing may be controlled in a livestock housing which incorporates the ventilation system of the present invention. Specific Description Figure 1 illustrates a heat exchanger usable in embodiments of the present invention, to recover heat from fluid which has been heated in the interior of a housing. As described above, embodiments of the present ventilation system may be used in a variety of different applications. In some embodiments, liquid-to-air heat exchange may be used for heat recovery, and in alternative embodiments air-to-air heat exchange may be used. For the purposes of illustration, the Figures will be described by reference to ventilation systems in which heat is exchanged between heated air which is being exhausted from inside the housing, and an incoming stream of supply air before it has reached the interior of the housing. Figure 1 schematically illustrates a heat exchanger 100 in which an incoming stream of supply air enters through a heat exchanger inlet A is routed through a heat exchanger along an incoming flow pathway 110 which is in thermal contact with an outgoing flow pathway 120. A supply fan 130 operates to draw the stream of supply air in through the heat exchanger, and to inject it into the interior of a housing (not shown) through a housing inlet B. At the same time, an exhaust fan 140 operates to extract a flow of exhaust air from a return duct C inside the housing, and drives it through the outgoing flow pathway 120 and out of an exhaust outlet D. A source of heat in the housing, for example heat-generating livestock or IT equipment, continuously heats the air inside the housing, such that the exhaust air is always at a higher temperature than the incoming supply air. As they pass through the heat exchanger pathways in opposite directions, heat is transferred from the hotter exhaust air, through a wall of the heat exchanger 100 into the stream of cooler supply air, so that the supply air is warmed and recovers heat which would otherwise be lost when the heated air is exhausted from the housing. The supply fan 130 may be a variable-speed fan, and the exhaust fan 140 may be a variable-speed fan. By varying the fan speeds of one or both of these fans, the residence time of the supply air and the exhaust air may respectively be varied, and the amount of heat transferred between the heat exchanger pathways can thus be controlled. Slower fan speeds typically mean that the airflows spend more time in the heat exchanger so that the supply air in turn recovers more heat from the exhaust air, but a minimum flow rate of supply air may be required to prevent the interior of the housing from overheating. The heat exchanger acts as a heat recovery system which uses a heat exchange process to transfer heat from one liquid or gas to another liquid or gas. This can be a direct process where the fluids are only separated by a heat transfer surface, or this can be an indirect heat exchange process where an intermediate fluid is used to transport heat. Heat exchanger configurations include contra-flow, concurrent flow and cross flow, and heat exchangers designs include plate and tubular construction methods. A variety of heat exchanger designs known in the art are usable in ventilation systems of the present invention. Figure 2A schematically compares performance characteristics of a variable-speed supply fan usable with a heat exchanger in embodiments of the present ventilation system. A dotted line 200 illustrates that the heat value provided to the interior of a housing by the heat recovered in a heat-exchanger powered by a variable-speed fan varies linearly with fan speed. In terms of heat recovery, it would therefore be beneficial to operate the variable-speed fan at its maximum speed. A dot-dash line 210, however, illustrates that the economic cost of running a variable-speed fan at different speeds worsens significantly at higher speeds, so that as an example the energy and financial cost of running the fan at its maximum (100%) speed is approximately four times higher than running the fan at 60% of its maximum speed. Taking this cost penalty into account and balancing it with the benefit of the heat recovered by the fan driving a heat exchanger, the overall efficiency of a ventilation system driven by this variable-speed fan peaks when the fan is operating at around 65% of its maximum speed. The solid line 220 shows that the economic benefit of running the variable-speed fan to drive a heat exchanger is highest at this fan speed, and then declines at higher fan speeds due to the increased electricity consumed by the fan at such speeds. In the ventilation system of the present invention, this difference in optimum operating conditions can be harnessed to run a ventilation system at the conditions that are optimal for the preferences of a given operator. Figure 3 illustrates the energy consumption, economic cost and carbon cost of operating a variable-speed fan in embodiments of the present invention. In the present ventilation system, the controller is programmed to monitor the energy consumption of the variable-speed fan(s) driving the heat exchanger, as well as the energy consumption of any single-speed fans in the system, and the energy consumption of an additional heater which can be operated to directly heat the air inside the housing. The controller can control the amount of heat recovered in the heat exchanger by controlling the running speeds of the fans, as well as controlling the supply of additional heat by turning on the heater, and / or varying the heat output of the heater. The controller receives temperature data from an inside temperature sensor which senses a temperature of the air inside the housing, and a supply air temperature sensor which senses a temperature of the supply air, preferably before it passes through the heat exchanger. From this, and a target temperature of the housing which may be preprogrammed or set by a user, the controller calculates the input heat requirement of the housing, and then controls the operation of the fans and heater to achieve that input heat requirement in one of several ways. The controller is preferably configured to receive data on energy prices and the carbon footprint of energy consumed by the ventilation system. The system may in many cases consume energy from multiple different sources - for example electricity to power fans and gas to power the heater. The system may be configured to draw electricity from renewable sources such as solar panels or wind power, and to draw electricity from the mains as required. The economic cost and carbon footprint of these various sources of energy may differ greatly, so the controller may advantageously factor in these parameters in order to control the ventilation system in line with the user’s priorities. The performance characteristics of any given system will depend on the type of energy used by the fans and / or heater. As shown schematically in Figure 3 for illustration, however, the operating conditions of a given ventilation system may create different performance curves for overall energy consumption (dashed line 310), economic cost (dash-dot line 320) and carbon footprint (dotted line 330). The optimum operating conditions for the fans and heater may be completely different if the system owner wants to minimise cost (dash-dot line 320) than if the system owner wishes to minimise the overall carbon footprint of the system (dotted line 330). The controller of the present ventilation system can advantageously be set to operate in any one of a number of different control modes. In each of these control modes, the controller may advantageously operate the system in the way that optimises for specific preferences, while still providing the required heat input to keep the interior of the housing at the required temperature conditions. For example the ventilation system may be operated in an energy control mode which minimises electricity consumption, or the ventilation system may be operated in a cost-saving mode that minimises the overall financial cost of operating the system, or the ventilation system may be operated in a carbon-saving mode that minimises the overall carbon footprint of operating the system, or ventilation system may be operated in an energy-recovery maximising mode that maximises the amount of thermal energy that is recovered by the heat exchanger. Figure 4 is a schematic diagram illustrating system controls usable by a controller in a ventilation system according to an embodiment of the present invention. As shown, the controller may be arranged to receive input data on input air humidity and temperature and exhaust air humidity and temperature. The controller may then control the input (supply) fan speed and extraction (exhaust) fan speed to dictate the heat exchanger efficiency. In systems containing a heater, the controller may also control the on-time and / or the heat output of the heater. By programming the controller with a process optimisation algorithm, the controller of the present ventilation system can advantageously control a variety of different parameters, and use those parameters to manipulate the overall energy consumption, carbon equivalent consumption and economic operating cost of the ventilation system. By setting the controller to operate the system in a particular control mode, the controller can then operate the system at the settings that best suit the user’s objective. Figure 5 illustrates variables which are controllable by the controller in embodiments of the present invention. The variables which can be controlled by the controller include fan energy consumption, the carbon consumption of the heating, the carbon consumption of the fan power, the cost of the heating, the cost of the fan power and the quantity of heat recovery. Figures 6A and 6B illustrate alternative factors which may be prioritised by a given user. Depending on the commercial priorities of the user or operator, the controller may be operated to optimise the financial performance by minimising operating cost, to optimise the thermal performance by maximising the heat recovery, or to optimise the environmental performance by minimising the carbon impact. Figure 7A is a schematic illustration of the heat input factors which affect the heat balance in a typical livestock housing containing poultry. The production of poultry is based on the following sequence of climate control steps: Pre-heating of building, typically to >30C, in preparation for placement of chicks or eggs. This step is typically 2 to 5 days in duration. - A ventilation period where the air flow quantity is adjusted to maintain the following: • Removal of excess carbon dioxide produced as a result of bird respiration; • Removal of ammonia released from bird excrement; • Removal of excess carbon dioxide and other products of combustion where direct heating is employed. • This step is typically 35 to 45 days in duration. The input heat requirement of the livestock housing is calculated from the target temperature of the housing, minus the heat produced by the birds and building heat losses which are dependent upon levels of insulation and leakage, minus heat recovered by the incoming supply air in the heat exchanger. The details of the climate control process are chiefly dependent upon the age of the birds, the ambient temperatures and the target environmental standards in the building. Figure 7A illustrates the progression of the separate factors in the heat balance. During the pre-heating period, no heat is produced by the birds, but after hatching the amount of heat produced by the birds slowly increased during their growth period of 35 to 45 days. The heat losses from the building scale with the temperature difference between the interior and exterior of the housing, so are highest during the pre-heating period when the temperature is highest, and gradually lower as the interior temperature of the housing is reduced. The heating of incoming air in the heat recovery system is controllable by the controller, and can be reduced as the heating requirement of the system reduces. The net heating requirement of the ventilation system is highest during the pre-heating period, and gradually reduces as the birds produce more and more heat. At a certain point in the growth cycle, the heat produced by the birds becomes greater than the building losses and the housing reaches a zero heat requirement. As illustrated in Figures 7A and 7B, by improving the control of the heat recovery process as well as fan and heater control, the ventilation system of the present invention can bring forward the ‘balance point’ where no external heating is required, so that it occurs earlier in the growth phase of the crop. This creates significant energy and cost savings and 5 advantageously lessens the environmental impact of livestock farming while still maintaining the climactic conditions in the housing within a suitable range.

Claims

1. A ventilation system for delivering air to the interior of a housing, the system comprising:a heat exchanger configured to recover heat by heat exchange between an outgoing stream of heated fluid and an incoming stream of supply air;a variable-speed supply fan configured to draw the stream of supply air through the heat exchanger and into the housing;an inside temperature sensor configured to sense a temperature of the air inside the housing;a supply air temperature sensor configured to sense a temperature of the supply air; anda controller configured to receive temperature data from the temperature sensors, and to control the supply fan;in which the controller is configured to calculate an input heat requirement of the housing based on the temperature data received from the temperature sensors, and in which the controller is programmed to operate in an energy control mode which sets the speed of the variable-speed supply fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing.

2. The ventilation system of claim 1, in which the controller is configured to calculate the input heat requirement by comparing the inside temperature sensed by the inside temperature sensor with a target temperature for the interior of the housing.

3. The ventilation system of claim 1 or 2, in which the controller is configured to calculate a recovered heat value for the heat transferred to the incoming stream of supply air in the heat exchanger.

4. The ventilation system of claim 3, in which the controller is configured to control the recovered heat value by controlling the speed of the supply fan to determine a flow rate of the stream of supply air through the heat exchanger.

5. The ventilation system of any preceding claim, in which the controller is configured to calculate a supply air heat value which is provided to the interior of the housing by the stream of supply air by combining the sensed supply air temperature with therecovered heat value.

6. The ventilation system of any preceding claim, in which the controller is configured to calculate the speed of the variable-speed supply fan which minimises the electricity consumed by the system while meeting the input heat requirement of the housing, by comparing the supply air heat value with the input heat requirement of the housing, and calculating a minimum supply fan speed at which the supply air heat value will match the input heat requirement of the housing.

7. The ventilation system of any preceding claim, in which the outgoing stream of heated fluid is an outgoing stream of heated liquid coolant.

8. The ventilation system of claim 7, in which the interior of the housing contains IT equipment, and in which the outgoing stream of heated liquid coolant is heated by heat generated by the IT equipment during operation.

9. The ventilation system according to any of claims 1 to 6, in which the outgoing stream of heated fluid is an outgoing stream of heated exhaust air, in which the ventilation system comprises an exhaust fan configured to direct the stream of exhaust air out of the housing through the heat exchanger, and in which the controller is configured to control the exhaust fan.

10. The ventilation system of claim 9, in which the exhaust fan is a variable-speed fan, in which the controller is configured to individually control the speed of each fan, and in which the controller is programmed to operate in an energy control mode which sets the speed of the variable-speed supply fan and the variable-speed exhaust fan at a level which minimises the electricity consumed by the system while meeting the input heat requirement of the housing.

11. The ventilation system of claim 9 or 10, in which the system comprises a heater configured to heat air inside the housing; and in which the controller is configured to receive temperature data from the temperature sensors, and to control the supply fan, the exhaust fan and the heater.

12. The ventilation system according to claim 11, in which the controller is configured to calculate the input heat requirement of the housing by comparing the insidetemperature sensed by the inside temperature sensor with a target temperature for the interior of the housing, and calculating the required input heat from the heater and / or from recovered heat received by the supply air in the heat exchanger.

13. The ventilation system according to claim 11 or 12, in which in the energy control mode, the controller calculates the total energy consumed by the variable-speed fan and the heater, and the controller sets the speed of the variable-speed fan and the heat input of the heater at levels which minimise the total energy consumed by the system while meeting the input heat requirement of the housing.

14. The ventilation system according to any preceding claim, in which in energy control mode the controller is programmed to run the variable-speed fan at a speed of between 55% and 75% of its maximum speed, or between 60% and 70% of its maximum speed.

15. The ventilation system according to any preceding claim, in which the controller is programmed to operate the system in one of a plurality of control modes, and in which the energy control mode is one of the plurality of control modes.

16. The ventilation system according to claim 15, in which the controller is programmed to preferentially supply recovered heat or to preferentially supply heat from a heater, depending on the control mode selected.

17. The ventilation system according to claim 15 or 16, in which the controller is configured to operate in one of the plurality of control modes at a time in response to a user input indicating a user priority.

18. The ventilation system according to claim 15, 16 or 17, in which the plurality of control modes comprising:a cost-saving mode, in which the controller calculates the economic cost of electricity and gas consumed by the variable-speed fan and the heater, and in which the controller sets the speed of the variable-speed fan and an on-time or output level of the heater at levels which minimise the cost of the electricity and gas consumed by the system while meeting the input heat requirement of the housing.

19. The ventilation system according to claim 18, in which in cost-saving mode, the controller is programmed to calculate the fan speeds and the heater-on time required to meet the input heat requirement at the lowest economic cost, and to run the fans and the heater at those operating conditions.

20. The ventilation system according to claim 18 or 19, in which the heater is a gas heater and the fans are electric fans, and in which in cost-saving mode the controller is programmed to calculate the financial cost of providing the input heat requirement from the gas heater, or from heat recovered in the heat exchanger, or from a combination of the two, and to operate the system at the settings with the lowest cost.

21. The ventilation system according to claim 18, 19 or 20, in which in cost-saving mode the controller is programmed to run the variable-speed fan at a speed of between 40% and 65% of its maximum speed, or between 50% and 60% of its maximum speed.

22. The ventilation system according to any of claims 15 to 21, in which the plurality of control modes comprising:a carbon-saving mode, in which the controller calculates the carbon footprint of the electricity and / or gas consumed by the variable-speed fan and the heater, and in which the controller sets the speed of the variable-speed fan and the on-time or output of the heater at levels which minimise the carbon footprint of the electricity and / or gas consumed by the system while meeting the input heat requirement of the housing.

23. The ventilation system according to claim 22, in which in carbon-saving mode, the controller is programmed to calculate the fan speeds and the heater-on time required to meet the input heat requirement at the lowest carbon footprint, and to run the fans and the heater at those operating conditions.

24. The ventilation system according to claim 22 or 23, in which the heater is a gas heater and the fans are electric fans, and in which in carbon-saving mode the controller is programmed to calculate the carbon footprint of providing the input heat requirement from the gas heater, or from heat recovered in the heat exchanger, or from a combination of the two, and to operate the system at the settings with thelowest carbon footprint.

25. The ventilation system according to claim 22, 23 or 24, in which in carbon-saving mode the controller is programmed to run the variable-speed fan at a speed of between 30% and 50% of its maximum speed, or between 35% and 45% of its maximum speed.

26. The ventilation system according to any of claims 15 to 25, in which the plurality of control modes comprising:an energy recovery maximise mode, in which the controller sets the speeds of the supply fan and the exhaust fan at levels which maximise the recovered heat value for the heat transferred to the incoming stream of supply air in the heat exchanger.

27. The ventilation system according to any preceding claim, in which the controller is configured to receive live data on system electricity consumption and gas consumption.

28. The ventilation system according to any preceding claim, in which the controller is configured to receive live data on the cost of the electricity and / or gas supplied to the system.

29. The ventilation system according to any preceding claim, in which the controller is programmed to assign carbon values to electricity and / or gas consumed by the system.

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