WO2012162760A1

WO2012162760A1 - Compact desiccant cooling system

Info

Publication number: WO2012162760A1
Application number: PCT/AU2012/000634
Authority: WO
Inventors: Stephen David White; Mark Jared GOLDSWORTHY
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2011-06-02
Filing date: 2012-06-04
Publication date: 2012-12-06
Also published as: AU2012262681A1; NZ619471A

Abstract

A solid desiccant cooling system includes, means (13, 17) defining a first pathway (11) for air to be cooled and a second pathway (15) for regeneration air, and structure (14) retaining a mass of solid desiccant for cyclic movement between a first location (14a), in which the solid desiccant lies in the first pathway for dehumidifying the air to be cooled by adsorption of moisture to the desiccant, and a second location (14b) in which the solid desiccant lies in the second pathway for the regeneration air to take up moisture therein as water vapour. An air heater arrangement (40) is provided in the second pathway upstream of the second location for heating the regeneration air, and an air cooler arrangement (18), independent of the air heater arrangement, is provided in the first pathway downstream of the first location. An air delivery device (50) is coupled to both pathways whereby the device is operable to deliver air along both pathways from a common intake, wherein the pressure drop along the respective pathways is of a similar magnitude. Also disclosed are a method of operating a solid desiccant cooling cycle, and a control system for a solid desiccant cooling system.

Description

Compact desiccant cooling system

Field of the invention

This invention relates generally to solid desiccant cooling systems of the kind in which a mass of solid desiccant is cyclically moved between an active, position in which it dehumidifies an airflow and a regeneration position in which hot air is employed to evaporate the moisture from the desiccant. The usual approach involves a rotary desiccant wheel, and the dehumidified air is usually further conditioned by evaporative cooling prior to its admission to a space to be cooled.

Background of the invention

Solid desiccant cooling systems of the aforementioned kind have been proposed in a variety of configurations. In the basic arrangement, fresh (outside) air is dehumidified in a rotary desiccant wheel. In this near adiabatic drying process, the air is unavoidably warmed. A heat recovery heat exchanger is used to cool the warm dry air back down to near ambient temperature. The resulting pre-cooled, dry air stream is then further cooled to temperatures below ambient using an evaporative cooling process before it is introduced into the occupied space to provide the desired space conditioning.

Regeneration of the desiccant wheel is required to ensure a continuous drying process. Regeneration is achieved by passing hot air through one side of the desiccant wheel. Moisture removed from the desiccant wheel is exhausted with the regeneration air stream exiting the desiccant wheel.

Regeneration air can be sourced from the occupied space (return air) or from outside ambient (fresh air). Regeneration air is first evaporatively cooled before it is pre-heated in the heat recovery heat exchanger. This minimises the supply air temperature before the supply air evaporative cooling process and maximises the regeneration air temperature before it is further heated in a heating coil with externally supplied heat. Desiccant cooling is primarily found in commercial and larger-scale installations, especially where higher humidity is a significant issue, for example in supermarkets and ice-skating venues. The technology is not found in residential applications to any significant extent, notwithstanding a number of potential advantages: robustness, easy maintenance and efficient operation with low temperature heat such as that from roof- mounted solar collectors, Solar desiccant cooling systems have been evaluated in a number of publications (including S.D. White et al. "Indoor temperature variations resulting from solid desiccant cooling in a building without thermal back-up", International Journal of Refrigeration 32 (2009), 695-704; and Rowe et al. "Preliminary findings on the performance of a new residential solar desiccant air-conditioner", Proc. Eurosun 2010, Graz, Oct 2010).

The limited application of desiccant cooling systems has arisen from disadvantages of the basic arrangement described above. This process suffers from (i) high parasitic fan power consumption due to the large pressure drops across the desiccant wheel and heat recovery wheel, (ii) bulkiness (due to the presence of two fans to respectively drive air on the supply and regeneration sides), (iii) cost and (iv) unsuitability for autonomous cooling with an intermittent heat source (due to the inability to achieve significant cooling when heat is not available for regenerating the desiccant wheel).

It has been proposed to address these disadvantages, at least to an extent, by replacing the heat recovery heat exchanger, employed to cool the warm dry air on the supply side back down to near ambient temperature and to pre-heat the regeneration air, with an indirect evaporative cooler on the supply side. _/

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. It is an object of the invention to provide one or more modifications of solid desiccant cooling processes of the kind earlier described that at least in part overcome the aforedescribed disadvantages.

Summary of the invention

It has been realised that the earlier mentioned proposal to replace the heat recovery heat exchanger with an indirect evaporative cooler on the supply side presents an opportunity to substantially eliminate the pressure imbalances between the supply and regeneration sides of the desiccant cooling circuit thereby enabling the conventional pair of fans to be replaced with a single fan supplying air to both the supply and the regeneration sides. It has been further appreciated that one fan instead of two would reduce the bulk and cost of the system.

The invention accordingly provides a solid desiccant cooling system, comprising: means defining a first pathway for air to be cooled, and a second pathway for regeneration air; structure retaining a mass of solid desiccant for cyclic movement between a first location, in which the solid desiccant lies in said first pathway for dehumidifying said air to be cooled by adsorption of moisture to the desiccant, and a second location in which the solid desiccant lies in said second pathway for said regeneration air to take up moisture therein as water vapour; an air heater arrangement in said second pathway upstream of said second location for heating the regeneration air; ah air cooler arrangement, independent of the air heater arrangement, in said first pathway downstream of said first location; and an air delivery device coupled to both of said pathways whereby the device is operable to deliver air along both of said pathways from a common intake, wherein the pressure drop along the respective pathways is of a similar magnitude.

The invention also provides a method of operating a solid desiccant cooling cycle, comprising cyclically moving a mass of solid desiccant between a first location, in which the solid desiccant lies in a flow of air and dehumidifies that air by adsorption of moisture to the desiccant, and a second location in which moisture is taken up from the desiccant by heated regeneration air, and delivering both said flow of air and a flow of said regeneration air from a common intake, wherein the pressure drop along the respective flows is of a similar magnitude. The coupling of the air delivery device to both of said pathways may include a flow divider at which respective fractions of the air are delivered to the respective pathways.

The air cooler arrangement may include an indirect evaporative cooler. A second, direct, evaporative cooler stage and/or refrigerative cooling stage, downstream of the indirect evaporative cooler, can also be optionally included. The air heater arrangement may include a device adapted to heat the regeneration air by "low grade" heat, e.g. one or more of a solar collector system, a solar hot water system, a heat pump, and an engine jacket coolant, either directly, or indirectly via an intermediate heat transfer fluid.

The air delivery device is advantageously an air circulation fan. The structure retaining a mass of solid desiccant is preferably a desiccant wheel.

The solid desiccant cooling system may include damper arrangements^, for selectively bypassing the mass of solid desiccant in the first pathway and/or diverting the heated regeneration air from the second pathway, and a controller arranged or programmed for selecting among these options. The system is thereby adaptable to be operated selectively in plural modes with respect to an associated space, for example desiccant cooling, non-desiccant cooling and heating. Preferably, the regeneration air does not include any air from the space to which the dehumidified air is directed. This minimises duct work, facilitates building internal pressurisation, and alleviates possible problems with positioning of the desiccant cooling process. The invention also provides a control system for the abovedescribed solid desiccant cooling system, comprising one or more damper arrangements for selectively bypassing the mass of solid desiccant in the first pathway and/or diverting the regeneration air from the second pathway, and a controller arranged or programmed for operating the damper arrangement(s) to selectively operate the solid desiccant cooling system in plural modes with respect to an associated space, which modes include desiccant cooling, non-desiccant cooling and heating.

The control system preferably carries out the method of the invention in an optimal operating mode such as space heating, indirect evaporative cooling, or desiccant cooling modes. Preferably the control system comprises at least four dampers. As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Brief description of the drawings

The invention will now be further described by way of example only by reference to the accompanying drawings, in which:

Figure 1 is a diagram of an air-conditioning configuration incorporating a solid desiccant cooling system according to a first embodiment of the invention;

Figure 2 is a diagram similar to Figure 1 of an air-conditioning configuration incorporating a solid desiccant cooling system according to a second embodiment of the invention; Figure 3 is a flowchart of logical steps for selection of an optimal operational mode for the configuration of Figure 2; and

Figure 4 is a 3-day log of relevant control inputs and the resulting control signal for the system of Figure 2. Detailed description of the embodiments

In the air conditioning configuration illustrated in Figure 1, fresh (outside) air 12 in a first pathway 11 defined by ducting 13 is dehumidified in one side 14a of a cyclic desiccant structure 14 such as a rotary desiccant wheel. In this near adiabatic drying process, the air is unavoidably warmed. An indirect evaporative cooler 18 is used to cool the warm dry air 16 in pathway 11 back down to near ambient temperature. The resulting pre- cooled, dry air stream 20 is then further cooled to temperatures below ambient using an evaporative cooler 22 before it is introduced into the occupied space 26 to provide the desired space conditioning.

Regeneration of the desiccant wheel 14 is achieved by passing hot air 28 in a second pathway 15 defined by ducting 17 through the other side 14b of the desiccant wheel. Water vapour evaporated from the desiccant wheel is exhausted with the regeneration air stream 30 exiting the desiccant wheel in pathway 15.

Regeneration air 27 is heated in a heating coil 40 with externally applied heat to obtain hot air 28 for regeneration of the desiccant wheel 14. Desiccant wheel 14 retains a mass of solid desiccant for cyclic movement, by rotation of the wheel, between first location 14a, in which the solid desiccant lies in pathway 11 for dehumidifying the air 12 to be cooled by adsorption, and second location 14b in which the solid desiccant lies in pathway 5 for the regeneration air 28 to take up moisture therein as water vapour. A single air circulation fan 50 pressurizes fresh ambient air 52 for the process and delivers it along both pathways 11, 15 from a common intake 54 at the fan. Thus, at a flow divider 56, one fraction 27 of the pressurised air is diverted, along pathway 15 defined by ducting 17, to heating coil 40 where it is heated and then used, as heated airflow 28, to regenerate the desiccant wheel.

The remaining fraction 12 of the pressurised air exiting fan 50 is delivered along pathway 11 defined by ducting 13 to the dehumidifying side 14a of the desiccant wheel where, as already described, it is first dehumidified and then cooled in turn by indirect evaporative cooler 18 and direct evaporative cooler 22.

The pressure of the air required from the fan is reduced, compared with the conventional process, through the elimination of the conventional heat recovery heat exchanger. Furthermore, the pressure drop over the regeneration air side is well matched with the pressure drop over the supply air side, i.e. the pressure drops are of similar magnitude and hence a single fan can provide air at a single pressure level suitable for both sides of the desiccant process. These factors lead to reduced parasitic fan power consumption.

By "similar magnitude" in relation to the pressure drops is meant that the difference between the pressure drops is preferably less than 60Pa, more preferably less than 30Pa and most preferably less than 10Pa. The differences in the pressure drop are typically related to differences in the length, diameter and/or configuration of the respective pathways. In preferred embodiments, in which the solid desiccant cooling system is used for residential applications, the pathway lengths are small (e.g. <1m) and, as such, the pressure drops across these respective pathways are expected to similar, if not the same.

By way of exemplification, in the conventional process employing a heat recovery heat exchanger, the pressure required at the supply (cooling) side is of the order of 300Pa, but the regeneration air must attain 420Pa or so. In the arrangement of Figure 1 , the cycle requires 320Pa on both sides and hence the inventors have realised that this is well balanced and suitable for use of a single fan to provide air to both sides of the process. Air pressure and associated parasitic fan power can be further reduced, for a substantial portion of a given year's operation, by operating in an alternative mode where the desiccant wheel is bypassed and cooling is achieved by indirect and/or direct evaporative cooling only. In this mode, air is not required for regeneration of the desiccant wheel.

This approach requires a new control system which preferably comprises a controller and switching damper devices as illustrated in the modified embodiment of Figure 2. Figure 2 also illustrates switching devices for enabling a separate winter space-heating mode of operation as described below. Four switching devices are provided. Damper 61 controls admission of supply side air 12 to the desiccant wheel, while damper 62 controls a bypass 70 of the desiccant wheel. Damper 63 is immediately upstream of the desiccant wheel in the pathway 5, while damper 64 controls diversion of heated air, downstream of heater 40, as space- heating air to occupied space 26. Closing and opening of dampers 61-4 is managed by a controller 80, which is configured or programmed to allow selection of various damper position combinations to set desired operating modes including desiccant cooling, non-desiccant cooling (in this case indirect evaporative cooling) and space heating. The selection may be by manual override but is normally in response to various environmental data inputs. Table 1 sets out damper positions for the three modes.

Table 1

The logic that determines the choice of optimum operation mode from data inputs is illustrated as a flowchart in Figure 3. The outdoor ambient relative humidity signal can be directly measured and supplied to the controller. A threshold outdoor relative humidity, below which there is limited advantage in using desiccant cooling (compared with indirect evaporative cooling), is around 50%.

It is also possible to use a number of alternative measured signals which indirectly infer the outdoor relative humidity and hence provide an approximate substitute. For example a time clock can be used to infer typical approximate diurnal variations in outdoor relative humidity. The temperature at the outlet of the desiccant wheel could also provide an approximate alternative to a direct outdoor relative humidity signal.

An examplary operating profile of the desiccant cooler of Figure 2, with the desiccant wheel regenerated by a solar thermal heat source, is illustrated in Figure 4. The period covers three days in summer.

In days 1 and 2, the desiccant cooling system is operating predominantly in desiccant cooling mode during daylight hours as (i) the hot water heat supply from the solar hot water system is at sufficient temperature and (ii) the outside relative humidity is above 50%. In the evening, stored heat in the hot water tank is depleted and the system goes into indirect evaporative cooling mode.

On the third day, the outside temperature is high, but the relative humidity is low. As a result, the system operates predominantly in indirect evaporative cooling mode, even though the hot water temperature is hot enough for desiccant cooling.

Year-long hour by hour TRNSYS simulations suggest that a solar desiccant cooling system, based on this design, would operate in indirect evaporative cooling mode more than 50% of the total operating hours in cooling mode.

In a modification of the configuration illustrated in Figure 2, an additional controlled portion of recirculation air, from the building into which the conditioned air is being directed, may be introduced into the air stream 16 that is passed through the indirect evaporative cooler 18. A suitable introduction point for this building recirculation air is shown at 71 in Figure 2. More generally, the invention envisages that there may well be additional cooling devices and/or circuits in the building or in the air circulation streams. It is believed that the inventive configuration, at least in one or more embodiments, is adaptable as a low-cost compact cooling system suitable for residential applications.

Notable advantages include:-

• A low capital cost, more compact system due to the reduced number of equipment parts. · Low air pressure drop and hence low parasitic fan power consumption.

• Ability to provide at least partial cooling in indirect evaporative cooling mode even when heat is not available. This makes it a more suitable year round cooling device, particularly for intermittent solar applications.

Claims

1. A solid desiccant cooling system, comprising: means defining a first pathway for air to be cooled, and a second pathway for regeneration air; structure retaining a mass of solid desiccant for cyclic movement between a first location, in which the solid desiccant lies in said first pathway for dehumidifying said air to be cooled by adsorption of moisture to the desiccant, and a second location in which the solid desiccant lies in said second pathway for said regeneration air to take up moisture therein as water vapour; an air heater arrangement in said second pathway upstream of said second location for heating the regeneration air; an air cooler arrangement, independent of the air heater arrangement, in said first pathway downstream of said first location; and an air delivery device coupled to both of said pathways whereby the device is operable to deliver air along both of said pathways from a common intake, wherein the pressure drop along the respective pathways is of a similar magnitude.

2. A solid desiccant cooling system according to claim 1 wherein the coupling of the air delivery device to both of said pathways includes a flow divider at which respective fractions of the air are delivered to the respective pathways.

3. A solar desiccant cooling system according to claim 1 or 2 wherein the air delivery device is an air circulation fan.

4. A solid desiccant cooling system according to claim 1 , 2 or 3 further including one or more damper arrangements for selectively bypassing the mass of solid desiccant in the first pathway and/or diverting the heated regeneration air from the second pathway, and a controller arranged or programmed for selecting among these options, whereby the system is adaptable to be operated selectively in plural modes in respect to an associated space.

5. A solid desiccant cooling system according to claim 4 wherein said modes include desiccant cooling, non-desiccant cooling and heating.

6. A solid desiccant cooling system according to any one of claims 1 to 5 wherein the air cooler arrangement includes an indirect evaporative cooler.

7. A solid desiccant cooling system according to claim 5 wherein the air cooler arrangement further includes a second evaporative cooler stage comprising a direct evaporative cooler stage, and/or a refrigerative cooling stage, downstream of the indirect evaporative cooler.

8. A solid desiccant cooling system according to any one of claims 1 to 7 wherein the air heater arrangement includes a device adapted to heat the regeneration air by "low grade" heat.

9. A solid desiccant cooling system according to claim 8 wherein the "low grade heat" comprises one or more of a solar collector system, a solar hot water system, a heat pump, and an engine jacket coolant, wherein the regeneration air is heated either directly, or indirectly via an intermediate heat transfer fluid.

10. A solid desiccant cooling system according to any one of claims 1 to 9 configured whereby in operation the regeneration airflow does not include any air from the space to which the dehumidified air is directed.

11. A method of operating a solid desiccant cooling cycle, comprising cyclically moving a mass of solid desiccant between a first location, in which the solid desiccant lies in a flow of air and dehumidifies that air by adsorption of moisture to the desiccant, and a second location in which moisture is taken up from the desiccant by heated regeneration air, and delivering both said flow or air and a flow of said regeneration air from a common intake, wherein the pressure drop along the respective flows is of a similar magnitude.

12. A method according to claim 11 including selectively causing said flow of air to bypass the mass of solid desiccant when the mass is at the first location and/or causing the heated regeneration air to be diverted before reaching the second location, whereby to operate the solid desiccant cooling cycle selectively in plural modes.

13. A method according to claim 12 wherein said modes include desiccant cooling, non-desiccant cooling and heating.

14. A method according to claim 11 , 2 or 3 wherein the dehumidified air is passed through an indirect evaporative cooler, downstream of said first location.

15. A method according to any one of claims 11 to 14 wherein said heated regeneration air is obtained by heating regeneration air with "low grade" heat.

16. A method according to claim 15 wherein the "low grade heat" comprises one or more of a solar collection system, a solar hot water system, a heat pump, and an engine jacket coolant, wherein the regeneration air is heated either directly, or indirectly via an intermediate heat transfer fluid.

17. A method according to any one of claims 11 to 16 wherein the regeneration air does not include any air from the space to which the dehumidified air is directed.

18. A control system for a solid desiccant cooling system according to any one of claims 1 to 10, comprising one or more damper arrangements for selectively bypassing the mass of solid desiccant in the first pathway and/or diverting the heated regeneration _» air from the second pathway, and a controller arranged or programmed for operating the damper arrangement(s) to selectively operate the solid desiccant cooling system in plural modes with respect to an associated space, which modes include desiccant cooling, non-desiccant cooling and heating.

19. A control system according to claim 18 wherein the damper arrangement includes at least four dampers.