Combined heat pump and ventilation system

Abstract

A heat pump for delivering heated air into a space wherein the same fresh air is used as the working fluid in a heat-pumping cycle. Outside air 0 may be drawn into an expander 1 in the heat pump cycle

Description

Field of the Invention: This invention pertains to the use of heat pumping for space heating. It also relates to space ventilation because the method of delivering net heat is to replace stale air from a space with warmed fresh air. Background: At present, much of the space heating done in the UK and elsewhere is achieved by burning a fuel such as oil or gas. The imperative to make dramatic reductions in the amounts of CO2 being released into the atmosphere is just one reason why it is desirable to change the way in which we heat spaces. The spaces in question comprise homes, offices, schools, hospitals, public buildings, greenhouses and workplaces as well as any other space that must maintain a reasonable temperature for the comfort and wellbeing of the people, animals or plants that may occupy them. Burning hydrogen is one possibility that has been much discussed and this may be an excellent solution in some locations - especially where geological deposits of naturally occurring hydrogen have been found relatively close to the spaces of interest. There are good reasons to believe that combusting hydrogen will not be a solution in the vast majority of locations. Improving the thermal insulation of the envelope where space heating is required is another measure much discussed. This clearly has the potential to have a very positive impact on reducing heating requirements very significantly. If thermal insulation of a space envelope was sufficiently good, then the natural heat emissions from living creatures could be enough to maintain the space temperature at a suitable level. It is not practicable in many cases to have the insulation quite this good for all possible external conditions. The present invention is also informed significantly by the requirement for ventilation in buildings. To prevent the build-up of moisture and odours in spaces, it is desirable that there should be a reasonable refresh rate of the air in those spaces. This refresh also has advantages in reducing the accumulation of infectious bacteria or virus units in a space. Modern building regulations call for a minimum refresh rate of air within spaces that is typically in the order one complete air-change every three hours. If the external temperature is significantly lower than the internal temperature, then the action of changing the air in a space on a regular basis has implications for heat loss unless some heat recovery heat exchanger is used to transfer most of the heat from the outgoing air into the incoming air. Such heat recovery heat exchangers are now commonplace and one such heat recovery heat exchanger features in the present invention. The system now most commonly proposed for decarbonising heating comprises heat pumps. Most heat pumps are designed to draw in electricity and they use that electricity to suck some heat from outside the space so that heat may be injected into the space that is the sum of the electrical energy input and the heat sucked from without. Every refrigerator is a heat pump, so the concept of a heat pump is actually a familiar one to almost everybody - albeit in a slightly different context. The "Coefficient of Performance" (CoP) for a heat pump describes the ratio between the heat that is injected into the space and the electricity that is consumed can be 5 or more - depending on the temperature at which heat must be delivered into the space and the temperature at which heat can be drawn from outside the space. As that difference in temperatures becomes larger, the CoP for a heat pump declines. If there is a very large temperature difference, then unless the heat exchangers and the machinery (especially the compressor) within a heat pump have very near-ideal performance, the CoP can drop to unity (i.e. 1) - meaning that we achieve only lkWh of heating for every lkWh of electricity put in. In these extreme circumstances, the "heat pump" behaves exactly like a simple resistor - turning electrical power directly into heat. Clearly, heat pumps provide an excellent solution for space heating. The most easily installed heat pumps are air-source heat pumps - where the external heat is drawn from the outside air. When the external temperatures are very low, the CoP of air-source heat pumps falls and the heat requirement for spaces also rises. Hence there is a doubly-bad effect on the demand for electricity. The electricity transmission and distribution system must be developed to cater for circumstances where external temperatures may fall severely. It is a mistake (made by many) to think that the most important measure of performance of heat pumps is the average CoP. From the perspective of an energy system, a strong emphasis should be placed on the CoP of heat pumps in the coldest weather. The present invention is inspired by addressing the combined requirements for good ventilation in spaces and for very good performance (i.e. very good CoP values) even in the coldest of weather conditions. The most common arrangement proposed at present for heat pumps operating in the UK is that the heat pump directs heat into water (or another liquid heat transfer medium). Then this liquid carries that heat into a set of heat transfer devices (in the UK, these are typically "radiators" or "convective heaters" or underfloor heating arrays) so that the heat transfers ultimately into the air in the space of interest. The requirement to transfer heat from water into the air drives the need for the water temperature to be higher than the required air temperature by some margin. This invention avoids one unnecessary heat transfer process because it raises the temperature of incoming air directly. One of the other features of the established heat pump designs is that they all employ a refrigerant of some sort. The "vapour-compression" heat pump cycles normally used rely on a two-phase process where the refrigerant is at least partially liquid as it picks up heat from the external source. The choice of refrigerant tends to be a compromise between three different considerations: cost, flammability and harmfulness to the environment. In the present invention, the only working fluid is air. The present invention draws some inspiration from an invention made by Lord Kelvin in the middle of the 19th century that he referred to as "Heat Multiplier". In this invention, Lord Kelvin proposed that air would be inducted from the atmosphere and expanded first such that the air temperature falls. Then this same air would draw heat via a heat exchanger before it is compressed again and discharged into the space. Lord Kelvin's patent was of its age. The proposed prime-mover providing the mechanical work was a steam engine and the heat losses from this steam engine were to be added to the heat contribution of the device. His thinking evidently did not consider the fact that air pushed into a space will necessarily require that a similar amount of air should leave that space. The present invention can be regarded as an evolution of Lord Kelvin's original concept wherein the prime mover might be an electrical machine and wherein a vital additional detail is added to enable the recovery of heat from air being exhausted from the space of interest. Description of the figures: Two different figures are presented. Figure 1 represents the space to be heated as a house and it represents the air external to the space of interest (0) using a cloud. Five separate devices are shown in this figure. These are: 1 An expander (1): When the system is configured for heating, this expander inducts air at a pressure slightly below ambient pressure and exhausting air at a lower pressure (named "manifold pressure" here) 2 A prime-mover (2) that would most commonly be an electrical machine supplied with electricity from a local electricity grid 3 A compressor (3): When the system is configured for heating this compressor draws air from the manifold and raises the pressure of this air back up to slightly above ambient pressure so that the air can be pushed into the space of interest 4 A first heat exchanger (4) arranged such that air in the manifold can exchange heat with the external air. When the system is configured for heating, this first heat exchanger draws in heat from the external air and transfers it to the air in the manifold. 5 A second heat exchanger (5) arranged such that air leaving the space can exchange heat with air in the manifold that is about to enter the same space. Figure 1 shows that items taken together, the expander (1), the primer-mover (2) and the compressor (3) constitute a powered compander (123). Figure 2 outlines one possible construction of a compander (represented schematically in Figure 1 as the powered compander (123)) based on the use of rolling membranes. Figure 2 shows a common mechanical connection (11) ensuring that the expander moveable part (12) will move together with the compressor moveable part (13). The expander moveable part (12) drives the expander rolling membrane (15) that is contained with the expander cylinder (18). The compressor moveable part (13) drives the compressor rolling membrane (14) that is contained within the compressor cylinder (19). The expander rolling membrane (15) defines a large part of the boundary of the expander chamber (17) at any one time. Correspondingly, the compressor rolling membrane (15) defines a large part of the boundary of the compressor chamber (16). A controllable intake valve (20) allows air into the expander chamber. A one-way exhaust valve (21) allows air to escape the compressor chamber (16) when the pressure within that volume exceeds ambient pressure. Four items like within the manifold connecting the expander chamber (17) to the compressor chamber. Two of these items ((22) and (25)) are controllable valves. A simple blower (23) helps to motivate air to leave the expander chamber (17) via the manifold to enter the compressor chamber (16). One or more heat exchangers are represented by the heat exchanger set (24). Description of the invention: The central concept of this invention is represented by Figure 1. In this system, incoming air (fresh air) is drawn into an expander (1) from the outside air mass (0) where its pressure is dropped before it leaves the expander (1) at a lower pressure entering a manifold or duct. The fact that work is withdrawn from the expander (1) means that the fresh air leaving this device in the manifold or duct has a temperature significantly below the temperature of the outside air mass (0). The fresh air leaving the expander (1) through the manifold or duct passes through a first heat exchanger (4) where it collects low-temperature heat from the environment. In some embodiments of the invention, it would collect this low-temperature heat from the outside air mass (0). In other embodiments, it could collect this low-temperature heat from a steam of water of from the ground. The concept of the invention is not dependent on the source of this low-temperature heat. Note that the pressure of the fresh air within the first heat exchanger is below ambient pressure - typically by a factor between 1.1 and 1.3. Having passed through the first heat exchanger (4), the fresh air then passes further along the manifold or duct through the second heat exchanger (5) where its pressure remains below ambient pressure. In the second heat exchanger (5), heat is passed to the fresh air from outgoing air emerging from the space to be heated. The second heat exchanger (5) would ordinarily be implemented as a contra-flow heat exchanger so that a high value of effectiveness is achieved. The outgoing air emerges from this second heat exchanger (5) at a temperature similar to the temperature of the outside air mass (0). The fresh air then passes through the compressor (3) its pressure rises back to just above ambient pressure so that it can be blown into the space to be heated. Most of the work required to drive the compressor (3) is drawn from the expander (1) but some additional work is also required because (a) there are invariably some pressure losses associated with any steady flow of air and (b) the air passing through the compressor (3) lies in a temperature range above the temperature range of air passing through the expander (1). The required additional work is supplied by the prime-mover (2). Most commonly, that primemover will be an electrical machine. The net amount of heat being delivered into the space is the product of three terms: (i) the mass flow rate of warmed air being delivered into the space (ii) the temperature difference between the warmed fresh air being delivered into the space and the temperature of the (stale) air being extracted and (iii) the specific heat of the air (approximately 1050 J / kgK). If high amounts of heat are required to sustain the desired temperature within a space, this will require either a high mass flow rate of air or a high difference between air delivery temperature and the desired temperature - or both. Having a high delivery temperature of the fresh air would defeat one of the primary purposes of this invention which is to allow lower delivery temperature for the heat pumped into the space and hence to support high coefficients of performance even in the presence of low external temperatures. The performance of the heat pump element of this invention is critically important on the performance of the compander (123). In effect, the power losses in both the expander (1) and the compressor (2) must be small relative to the total power processed by each one. One possible structure for a compander is represented in Figure 2 based on the use of rolling membranes. The proposed compander construction integrates a reciprocating expander and a reciprocating compressor into a single object - a compander. A highly important and highly distinctive feature of this invention is that the object employs two different rolling membranes nested one within the other. The outer rolling membrane (15) serves to describe the deformable part of the exterior surface of an expansion chamber (17). The inner rolling membrane (14) serves to define the deformable part of the surface that separates the compression chamber (16) from the expansion chamber (17). The compression chamber (16) is surrounded by the expansion chamber (17). Work is put into the object by causing vertical reciprocating motion of a common platform (11) that is directly connected to both the expander piston (12) and the compressor piston (13). The expander piston (12) is joined to the outer rolling membrane (15) such that as that expander piston (12) descends, the outer rolling membrane (15) acts like a set of "piston rings" between the outer wall of the expansion chamber (18) and the expander piston. The outer rolling membrane (15) maintains a closed boundary while that the volume remaining in the expansion chamber (17) is reduced towards zero. Because the pressure in the expansion chamber (17) is generally below the external pressure, work is done by ambient pressure on the unit as the expander descends. The compressor piston (13) is joined to the inner rolling membrane (14) such that as that compressor piston (13) descends, the inner rolling membrane (14) acts like a set of "piston rings" between cylindrical guide structure (19) of the compression chamber (16) and the compressor piston. The inner rolling membrane (15) maintains a closed boundary while that the volume remaining in the compression chamber (16) is reduced towards zero. Because the pressure in the compression chamber (16) is generally higher than the pressure in the expansion chamber (17) that surrounds it, work has to be done on the compressor piston to cause it to descend. The cylindrical guide structure (19) for the compression chamber does not have to be impermeable. In most practical implementations this structure would have small gaps through the surface to facilitate the easy transfer of gas through this. This object sits within the expansion chamber volume (17) and it is not intended to be a boundary of this volume. Rolling membranes are invariably deployed such that the pressure difference across each one tends to put the surface of the rolling membrane into tension. The present invention respects this constraint. The details of how valving would be operated to cause this invention to operate successful will be obvious to any person skilled in the art. An intake valve (20) allows air (or another gas) into the expansion volume (17) at appropriate times. This intake valve (20) would open for a part of the upward stroke of the common platform (11) but it would close before the common platform (1) reached "top dead centre". An exhaust valve (21) allows air (or another gas) to be expelled from the compression volume (16) at appropriate times. This exhaust valve (21) would open for a part of the downward stroke of the common platform (11) but would open only after the pressure within the compression volume (16) was equal to the target outlet pressure. The expansion volume (17) and the compression volume (16) are in communication with each other via a gas transfer path. That path itself is not numbered in Figure 2 but it carries one compressor-side valve (22), a heat transfer section (24), one expander-side valve (25) and (optionally) a blower (23). The blower (23) may be used to ensure that there is always at least some positive pressure differential between the compression volume (6) and the expansion volume (17) so that the inner rolling membrane (14) remains mechanically stable. Advantages of the invention: One advantage of the present invention is that no refrigerants are involved. Air itself forms the working fluid by which heat pumping is achieved. A second attraction is that the system proposed combines two functions required for modern spaces - heating and ventilation. The same system delivers fresh air into the space of interest, removes stale air and recovers heat from that stale air prior to expelling it to the external environment. A third attraction of this system is that the heat being delivered into the space is carried by fresh air that will mix directly with the air already present. This contrasts with the normal mode of heat delivery from heat pumps where water (in a closed circuit) is heated to a temperature significantly higher than the target temperature for the space. A limitation of the present system is that if it is applied to spaces with poor insulation, the heat delivery requirement may be large and this may call for excessively-high delivery temperatures for the fresh air unless the mass flow rate of incoming fresh air is impractically high. Summary of the invention: The invention comprises an expander (1), a prime-mover (2) (that would normally be an electrically driven machine), a compressor (3), a first heat exchanger (4) configured such that the air in the manifold (the set of pipework joining the compressor (3) and the expander (1)) can exchange heat with the external air and a second heat exchanger (5) configured such that air emerging from the space of interest can exchange heat with the air in the manifold. The main operational mode of this invention is a heating mode wherein the objective is to introduce air into the space of interest at a temperature sufficiently higher than the target temperature within that space that the net influx of heat compensates the loss of heat from that space through the envelope (walls, floors, windows, doors and ceilings). The working fluid in this process is air itself. During a heating operation, air from outside of the space envelope to be heated (henceforth called "process air") is inducted into the expander (1) where this air is allowed to enter the manifold. Work is extracted from the process air travelling across the expander and that work is fed into the prime-mover (2). Because the pressure reduces in the expander (1), its temperature falls significantly below what it was for the original (atmospheric) pressure and this enable the process air to pick up heat from the external air in a first heat exchanger (4). The air within the manifold then passes through a second heat exchanger (5) where it recovers heat from air leaving the space being heated. After recovering this residual heat from the air leaving the space, the process air finally proceeds through a compressor which restores the pressure to a value slightly above ambient pressure so that the process air is motivated to pass into the air space within the space. This air entering the target space (to be heated) is what causes the other air to leave that space. One key distinguishing feature of this present invention over what has been presented previously is that a heat recovery heat exchanger (the second heat exchanger (5)) is in place to remove all (or nearly all) of the heat from the air leaving the target space before that air is exhausted back to ambient conditions external to the space. Note that the second heat exchanger (5) would invariably be configured in a "contra-flow" configuration so that the hot ends of both streams were located at the same end of the second heat exchanger (5) and similarly the cool ends of both streams were located at the same second heat exchanger (5). Thus we may regard that the present invention is a form of heat pumping system for heating some target space in which heat is introduced into the target space by passing in air at a temperature higher than the target space air temperature. Said heat pumping system comprises an expander, a prime mover, a compressor and two heat exchangers wherein the first heat exchanger exchanges heat with an external source of low-grade heat, the second heat exchanger recovers all (or most of) the heat from the air leaving the target space. The expander lowers the pressure of the process air (and correspondingly its temperature) such that the process air can collect heat from the external source. Subsequently, the compressor raises the pressure of air again such that it can be pushed into the target space at a pressure very similar to ambient pressure. The key differentiator between this invention and what has gone before is that two separate concepts are tied together to make a single system that achieves a better result than would be achieved by addressing these two concepts in isolation. The two concepts are: (i) performing heat pumping using air itself as the working fluid within the thermodynamic cycle and (ii) employing a heat recovery heat exchanger to recover heat from the outgoing air from a space and transferring that heat into the air in a manifold. In isolation, the two key concepts are very well established. The novelty in the present invention relates to the combination of these two concepts. Generalisations of the embodiment: Multiple generalisations of the basic embodiment described in Figure 1 are possible whilst remaining within the scope of the present invention. The first of these generalisations is that the source for the low-grade heat drawn from points external to the space of interest might sometimes be the ground (including the use of either shallow buried heat transfer pipes or deep vertical circuits) or it might be a river or stream of water or it might be latent heat pumping (turning water into an ice slurry and discarding that ice slurry down the drain). The same basic idea works in all of those cases. The second generalisation is that the compressor (3) and expander (1) could be turbo-machines rather than being a positive displacement machine as illustrated in Figure 2. Indeed, the normal interpretation of Figure 1 would be that the compressor (3) and expander (1) would be turbo-machines. The concept works well in both cases - turbo-machines for compressor (3) and expander (1) or positive-displacement machines. A third generalisation is that the system of Figure 1 can be run in reverse to cause space cooling at times when the ambient conditions have a higher temperature than is desirable in the space. Then the problem is to reject heat from the space and this would be done by drawing in air from the surroundings, compressing it first to raise the temperature of that air, then rejecting some heat into the surroundings via the first heat exchanger (4), then rejecting some further heat into air being exhausted from the space via the second heat exchanger (5) and finally expanding the process air such that it cools down to a temperature below the target temperature of the space before injecting that air into the target space. To run in reverse, it is necessary to re-route some air connections. In this mode, air from external conditions is first compressed before entering the manifold and expanded immediately before entering the target space. The sequence of operations in this mode will be obvious to any person skilled in the art.

Claims

(1) A heat pump for contributing heat into a space uses the same fresh air as both the working fluid in a heat-pumping cycle and a medium delivering the heat.(2) A heat pump for contributing heat into a space as described in claim 1 comprising an expander drawing air from outside the space, a primemover, a compressor driving warmed fresh air into the space, a manifold connecting the exhaust of the expander to the intake of the compressor, a first heat exchanger configured to transfer heat from an external source into air within the manifold and a second heat exchanger configured to transfer heat from air emerging from the space into air within the manifold.(3) A heat pump for contributing heat into a space as described in claim 2 in which the compressor and expander are mechanically commoned such that all of the mechanical work extracted by the expander was fed directly into the compressor so that the prime-mover supplies only the difference between these two work quantities.(4) A heat pump for contributing heat into a space as described in either of claims 2 or 3 in which second heat exchanger is configured in a contraflow sense such that both streams of air passing through it are warm at the same end and cooler at the opposite end.(5) A heat pump for contributing heat into a space as described in either of claims 3 or 4 in which both the expander and compressor are realised using rolling-membrane devices with a reciprocating action wherein the expander reduces air pressure during one half of one reciprocation and the compressor increases air pressure during the other half of one reciprocation.

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