WO2006095055A1

WO2006095055A1 - Phase change material heat exchanger

Info

Publication number: WO2006095055A1
Application number: PCT/FI2006/050092
Authority: WO
Inventors: Kari Moilala; Michael Gasik
Original assignee: Mg Innovations Corp.
Priority date: 2005-03-09
Filing date: 2006-03-07
Publication date: 2006-09-14
Also published as: FI20050252A0; FI20050252A

Abstract

The invention relates to a phase change material (PCM) heat exchanger consisting of a heat exchanger (1) operating on the countercurrent principle and of phase change material (PCM) 5accumulators (2) provided in the heat exchanger. When the directions of the air, gas and liquid flows are cyclically reversed in the apparatus, energy is recovered into the heat exchanger (1) and the PCM accumulator (2), and during the subsequent cycle energy is released from these. While one heat exchanger (1) and PCM accumulator (2) is being charged, the other ones are simultaneously discharged. The invention is applicable to buildings, vehicles, climate control rooms, single or 10multiple devices such as computers, and in various processes, such as but not limited to cryogenic methods, and also in space technology and supercritical carbon dioxide applications.

Description

Phase Change Material Heat Exchanger

This invention relates to a phase change material (PCM) heat exchanger consisting of a regenerative

5stationary heat exchanger operating on the counter-current principle and of a phase change material

(PCM) enthalpy accumulator in the heat exchanger. The system provides free pre-cooling and pre- drying of fresh ventilation air in the summer and for pre-heating and pre-moisturising of fresh ventilation air in the winter. In addition to buildings, the objects of use could include vehicles, industrial and commercial equipment rooms, as well as any closed compartments which require lOcontrolled climatic conditions, for instance for cooling in processes and apparatuses such as computers and telecom equipment. Besides air or gas cooling, the heat exchanger is applicable for enhancement of =heat transfer in liquids.

Efficient air ventilation should allow energy recovery from exhaust air flow, utilising moisture and pre- 15cooling/heating. These issues have a varying impact depending on the geographic zone concerned. In the cold climate zone, heat recovery is important, in the temperate zone cooling and moisture balance are given priority, whereas cooling and outdoor air dehumidification are preferable in the hot zone.

0On the global scale, energy is abundantly used for cooling, opposing the natural tendency of thermal energy to pass towards a cooler object. As the climate warms up, cooling and dehumidification will require more energy. Conventional compressor cooling is a highly energy-consuming method. The thermoelectric unit based on the Peltier effect converts electricity into cold, however, it incurs high production costs and excess heat at the hot side of the device, thus becoming inappropriate for 5objects requiring larger amounts of energy.

A condition for indoor air of good quality is that the total air amount is replaced from once to three times an hour. The target temperature for pleasant indoor air in residential buildings is 25⁰C, the relative humidity being 55%. 0

Besides in buildings, vehicles and industrial processes, temperature control is vital also in heat- generating devices, typically computers and industrial electronics.

Energy savings in living rooms are often given priority at the expense on ventilation, which yet is 5indispensable. This leads to lower mental agility, increased carbon dioxide levels, indisposition, moisture and mildew problems, allergic diseases and buildings attacked by mildew spores.

Introduced into the duct system, heated and humid air may cause mildew problems, since moisture and heat is all that mildew spores need for growing. Moisture condensation in the heat recovery cells also entails excess moisture problems.

Dehumidification of air is a more complicated and costly operation than mere cooling. One solution 5suggested for resolving this problem involved recirculation of indoor air that had been dried once. However, the lack of fresh air supply causes a problem in that case.

In a PCM heat storage, known per se, material phase changes are most frequent between a solid and a liquid states. Such storages are usually maintained in the temperature range 0-100 °C, being lOthus suitable for short-term energy storage when connected to heaters and coolers, among other things. Typical media comprise water/ice, salt brines, inorganic salt hydrates, saturated hydrocarbons and fatty acids of high molecular weight. PCM storage units have the benefits of a small size, compared e.g. to storage units for water alone, and of absence of any movable parts. PCM materials have recently been utilised for the heating and cooling of wearing cloths. One drawback of PCM

15storages is caused by their poor heat conductivity. PCM storages can also be given a plate-like shape. Heat discharges from the PCM storage constitute a major problem, because further heat cannot be stored unless it has first been discharged. The PCM operation is thus based on cyclic charges and discharges. One of the advantages of PCM materials is operation with small temperature differences. 0

The cooling demand in buildings depends on three components: the heat load caused by outdoor air, by indoor air and by ventilation. Heat recovery operating on the counter-current principle has proved to yield higher efficiency than a system operating on the forward-current principle. In a regenerative system, heat is stored effectively in heat recovery cells. 5

The warmer the air, the higher its potential water vapour content. However, at a given temperature, air can have only specific maximum water vapour content (absolute humidity). In that case, air is said to be saturated with water vapour. If the air temperature drops at that stage, the moisture surplus will condense in the form of liquid droplets from the air. The temperature at which water starts separating Ofrom the air is called the dew point. Since the energy balance from the cooled air must be maintained, excess latent enthalpy from condensed water is transferred to the condensation surface, thus heating this surface. On the other hand, as air gets warmer, moisture may evaporate in the air, and then the surface where evaporation takes place is cooled. This is called evaporative cooling, which can be observed in nature as perspiration on the skin, among other things. In conventional 5ventilation systems, moisture is detrimental and condensed excess water should be removed, because it freezes at temperatures below zero (in winter) or stacks in the air passages, causing flooding (in summer). In recuperative cross-current plate heat exchangers, air currents are not reversed, and hence they cannot interact optimally with a PCM storage nor with a regenerative rotating heat recovery cell.

A stationary, regenerative and accumulating storage cell system operating on the counter-current 5principle is straightforward and effective. The cell system may be made of any material having high thermal storage (heat capacity), such as aluminium or copper.

The invention has the purpose of eliminating or of at least of reducing the drawbacks mentioned above and of achieving a system, in which the ventilation thermal load is reduced to such a point lOthat, in the summer, air entering under the effect of natural evaporative cooling and of the PCM accumulator will get cooled even without any additional energy supply. At the same time, outdoor air humidity will be stabilised. The procedure is reversed in the winter. The invention is also applicable to heat transfer between liquids.

15Latent heat is not observed as temperature increase, since it is the energy required for a material to pass from one state to another, such as from ice to water and from water to vapour. Such state changes may be endothermic, i.e. they bind heat, or exothermic, i.e. releasing thermal energy. Thus, for instance, the energy required for water evaporation is released when vapour is re-condensed in the form of liquid water. 0

The enthalpy recovery apparatus of the invention is characterised by utilising the humidity contained in outdoor air, so that, when warm outdoor air passes through a cooler heat recovery cell, the air will be cooled, and being in a saturated state, it will be incapable of retaining moisture in the air, but instead, moisture will be condensed from the air, forming liquid droplets on the surface of the highly 5heat conductive heat recovery cell. This is precisely the moment when indoor air, instead of outdoor air, is conducted through the heat recovery cell, so that the indoor air is heated while passing through the cell, which has been heated by condensed water and warm outdoor air. At the end of the cycle, the air being sufficiently heated, it will be capable of binding moisture, with water evaporating from the cell surface and cooling the surface. At this stage, the air current direction is alternately reversed Oonce more, being directed from outdoors towards indoors, so that the outdoor air is cooled and the moisture is condensed. The condensed moisture forms a moisture film on the cell surface, and such a film is not condensed to harmful water. Unless the outdoor air has sufficient humidity, an additional evaporative cooling can be artificially provided by spraying moisture into the cell system.

5Compared to a conventional system, this new system yields two appreciable benefits: air is both cooled and dried without supplementary energy, but using alternating enthalpy charge/discharge principle. In the winter, the procedure is mirrored, when indoor air moisture is condensed on the surface of the cell system, and heat is absorbed. During the subsequent cycle, cold fresh air releases the heat from the cell and absorbs it. At the same time, the indoor air moisture level remains high. The cell system naturally needs to have an adequate active surface area for moisture to be adsorbed on its surface each time. Otherwise, a portion of the moisture will be condensed on other surfaces, such as the casing, where it does not evaporate during the following cycle, and the 5efficiency will decrease accordingly. Similarly, the enthalpy-absorptive mass should have adequate volume in order to be capable of storing the energy released during condensation cycle.

Besides high energy performance, such cyclic condensation and evaporation yield the benefit that impurities in the air (dust, particulate matter, etc.) do not permanently adhere to the surface of the lOcell system. For the same reason, the system has impeccable function in bakeries, whose ventilation conditions are usually difficult, without obstruction of the cell system.

The amount of water vapour contained in the air and the corresponding partial pressure of water vapour are important in terms of the heat insulation of mechanically cooled premises. With the air

15temperature remaining constant, but with a decreased relative humidity, the partial pressure of water vapour will drop accordingly. Moisture is transferred into the dwellings when the air of the premises is replaced, doors or windows are opened or during ventilation, and also by diffusion through the walls. Diffusion is due to the fact that the partial pressure of water vapour is higher outdoors than in the dwelling space.

20

The small temperature difference between indoor air and air reaching the evaporator of the cooling apparatus reduces the tendency of the moisture in the air to condense as water on the cool evaporator surfaces. When outdoor air enters mechanically cooled premises, it will increase the load on the refrigerating machinery, because, firstly, outdoor air needs to be cooled to a temperature

25below that of indoor air, and secondly, the liquefaction heat released when water vapour is condensed to water needs to be removed. Thirdly, the diffusion mentioned above generates an additional load for conventional cooling and air conditioning systems.

If the indoor temperature is 2O⁰C and the relative humidity is 55%, air contains 9.7 g/m³ of water

3Ovapour. If the outdoor air temperature is 34⁰C and the relative humidity is 100%, this air contains 39.5 g/m³ of water vapour. With a dry air specific heat of +0.31 kcal/m³ (at 2O⁰C) and a water vapour liquefaction heat of +0.61 cal/g, the additional thermal load increase on the refrigerating machinery caused by the outdoor air humidity drop is about 22.5 kcal/m³. When the heat recovery cell system of the invention is used, the corresponding increase is only about 9.7 kcal/m³, i. e. with a decrease of

3557%.

The enthalpy recovery ventilator achieves the following advantages, among other things (on the basis of the following tests VTT/4 October 1995; RTE10406/95 and SINTEF/1982-08-11/STF15 F 82029, 1982-05-28/150164): 1) Excellent temperature efficiency (VTT: 87.8%; SINTEF: 98%). 2) Under hot (above 30 C°) and humid (relative humidity above 80%) outdoor conditions, the apparatus cools indoor air by up to 3-5 C° without supplementary energy. 3) When correctly used, the heat recovery cell system does not freeze. 4) Low connection power. 5) Control of relative humidity in 5indoor air to 38-67%.

An apparatus having the properties described above is usually called a ventilation-heat recovery apparatus. A more appropriate name would be ventilation-enthalpy recovery apparatus, since, in addition to water, it utilises energy conversions relating to the various state changes of water, among lOwhich the latent heating required for evaporation is essential. This latent heating is utilised in evaporative cooling.

In accordance with the invention, heat and moisture loads are being reduced by adsorption without additional external energy in the enthalpy recovery cell system. The necessary supplementary

15heating and cooling is provided by the PCM enthalpy accumulator, with one or more PCM accumulators added to the heat recovery cell, the heat stored in the accumulator being released when the air flow directions are reversed. The apparatus comprises two enthalpy recovery cells, in which the air flow directions are alternately reversed. Heat conduction can be enhanced by devising the lamellas or other forms of the heat exchanger plates of the enthalpy recovery cells and the PCM 0material so as to generate a turbulent air flow. This can be achieved by providing lamellas and PCM material undulating in parallel with the air flow direction, and then the gas molecules are in effective contact with these surfaces, providing effective heat transfer. There may be a variety of PCM materials and lamellas, e.g. with one devised for a given temperature range and another for a range under or above this, so that, when the one stops operating, the other starts, or that the one has 5optimal operation in summer conditions and the other one operates optimally in winter conditions. All things considered, the exactly appropriate PCM materials can be tailor-made for different applications.

One of the main advantages gained by the invention is enhanced operation of the PCM accumulator 0by effective heat release. Heat release can be enhanced also with the use of a conventional liquid- circulation system, in which the heat can be transferred for heating service water, for instance. The invention is also applicable to heat transfer between gases. The system of the invention has higher efficiency than that of conventional cooling apparatuses.

5The apparatus of the invention is straightforward, inexpensive, relatively silent, light, maintenance- free, and it does not contain hazardous substances. In accordance with the invention, the energy is maintained at its original location, the heat loss is as low as possible. The invention involves a kind of energy recycling. This reduces energy consumption, because the obtained temperature is not changed immoderately. Owing to the high efficiency ratio, the temperature of fresh air is identical with or almost equal to the indoor air temperature. 5

In conventional ventilation, heat recovery devices and coolers have long payback times. In addition to the initial costs, traditional heat recovery ventilators and heat pumps utilising outdoor air cannot operate without additional external energy at temperatures below zero. Indoor air heat pumps, again, are of no use when the outdoor air is warmer than the indoor air. Consequently, they will have very lOshort operation periods per year (either in winter or summer, but not through the year). Should it be necessary to use both the systems, together with an air drier or humidifier, the costs would be even higher. Calculated in terms of the hot zones in the United States and of their average electricity tariffs, the ventilation device of the invention may have a payback time of less than a year. The excellent seasonal performance factor (SPF) is due to the long period of use each year, i.e.

15throughout the year in the practice, since the apparatus operates both in the winter and in the summer.

Different solutions, comprising methods known per se, have been developed for all these known purposes. This invention allows the problems described above to be solved by means of one single 0apparatus.

Various embodiments of the invention are defined in the dependent claims. The invention is explained below by means of an example and with reference to the accompanying drawing, which is a schematic view of a PCM heat exchanger and of its air, gas and liquid flows. 5

The PCM heat exchanger consists of at least two regenerative heat exchangers 1 operating on the counter-current principle, through which air, gas and liquid flows are directed to and from the apparatus with alternating and cyclically reversed opposite flow directions. The flows are indicated by means of arrows. The heat exchangers 1 are placed next to each other, spaced by a plate for

30preventing mixture of the air and liquid flows and thermal conduction. One or more plate-like cells made of phase change material, i.e. PCM accumulators (2) have been mounted in the heat exchanger 1. When hot air or liquid enters the cell (1), the cell releases its energy to the heat exchangers 1 and changes the state of the material in the PCM accumulator. This entails a temperature drop of the flowing air or liquid. The lamella of the heat exchanger may be made of a

35highly heat conductive material, such as aluminium or copper. The lamella and the PCM accumulator may be designed so as to generate a turbulent air or liquid flow, and then heat transfer will be effective; an undulating shape aligned with the air, gas or liquid flow is useful to this end. When sufficient heat has been recovered, the directions of the air and liquid flows are reversed, so that cold air/liquid is heated while passing through the warm cell system into a colder space. At the same time, heat is released into the air/liquid flow discharged from the PCM accumulator. The cyclic operation can be optimised in terms of the temperature, among other things. The water vapour contained in the air during such cycles is condensed onto the surface of the cell system, and during the subsequent

5cycle, it returns into the air by evaporation. The energy needed for evaporation is supplied from the cell system and the PCM accumulator, which are thus cooled. During the following cycle, the hot air is cooled when reaching the cold cell and the PCM accumulator, and at the end of the cycle, these two are also heated, resulting in the air current directions being reversed once more. The PCM accumulators operate with a small temperature difference, yet they need to be devised quite lOspecifically for a given object of use. Consequently, a plurality of accumulators may be provided in different temperature ranges, say, with one accumulator starting to operate when another has stopped, or with one operating in cold conditions while the other operates in warm conditions. PCM accumulators may be provided in only one 1 of the heat exchangers, however, higher efficiency is achieved with accumulators in both the heat exchangers, so that the one is continually charged while

15the other one is discharged. Heat can also be discharged from a PCM accumulator using a liquid circulation system, allowing heat to be utilised for other purposes as well, such as water heating. The invention is suitable in buildings and vehicles, but also for heat transfer in processes and devices, such as computers.

Claims

1. A phase change material (PCM) heat exchanger consisting of at least two heat exchangers (1), through which air, gas or liquid flows are directed to and from the heat exchanger (1) with their directions cyclically reversed within the device while remaining mutually opposite, characterised in that the heat exchanger (1) consists of at least one phase change material (PCM) accumulator (2).

2. A phase change material (PCM) heat exchanger as defined in claim 1, characterised in that a plurality of cyclically operating PCM accumulators are provided e.g. in different temperature ranges.

3. A phase change material (PCM) heat exchanger as defined in any of the preceding claims, characterised in that the phase change material (PCM) heat exchanger (1) is used in buildings, vehicles, devices such as computers, various processes, such as cryogenic methods, and in space technology and supercritical carbon dioxide applications.