NZ619471B2 - Compact desiccant cooling system - Google Patents

Abstract

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. 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 g system Field of the invention This ion relates generally to solid desiccant Cooling systems of the kind in which a in which mass of solid desiccant is cyclically moved between an activeposition 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 dified air is usually further conditioned by evaporative cooling prior to its admission to a Space to be .

Background of the invention Solid desiccant cooling systems of the aforementioned kind have been proposed in a variety of configurations; in the basic ement, 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 is then . near ambient temperature. The resulting pre-cooled, dry air stream 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. ration is achieved by passing hot air through one side of the‘ ant wheel. re d from the desiccant wheel is exhausted with the ration 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 preheated 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. 2 l Desiccant cooling is primarily found in commercial and larger-scale installations. especially where higher humidity is a significant issue, for example in superrnarketsand ice-skating venues.’ The technology is not found in residential applications to any significant extent, notwithstanding a number of potential ages: robustness, easy” ' maintenance and efficient operation with low temperature heat such as that from roof- mounted solar collectors, Solar desiccant cooling s have been evaluated in a number of publications (including 8.0. White et al. “indoor temperature variations resulting from solid desiccant cooling in a building without thermal back-up”, lntemational l of Refrigeration 32 (2009). 695-704; and Rowe et a]; “Preliminary ‘10 findings on the performance of a new ntial solar desiccant airbonditioner”, Proc.

Eurosun'2010. Graz, Oct ~ 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 theilarge pressure drops across the desiccant wheeland heat recovery wheel, (in 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 ity to achieve significant cooling when heat is not available for regenerating the desiccant wheel).

It has been proposed to address these disadvantages. at leastto an extent, by L2 20 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-h'eat the ration air, with an indirect ative cooler on the supply side. / ‘Reference to any prior art in the cation is not, and should not be taken as, an acknowledgment or any form of tion that this prior art forms part of the common l knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ained, understood and regarded. as relevant by a { I person Skilled in the art.

It is an object of the ion to provide one or more modifications of solid desiccant cooling processes of the kind earlier bed that at least in part overcome the escribed disadvantages, or to provide a useful alternative.

Summary of the invention It has been ed 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 ant cooling system, comprising: means defining a first y for air to be cooled, and a second y for regeneration air; structure retaining a mass of solid desiccant for cyclic movement between a first on, in which the solid desiccant lies in said first y 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, W0 2012/162760 PCT/AU20l2/Oil0634 wherein. the re drop along the respective ays ’is of a r magnitude.

The invention also providesa 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 ons of the air are delivered to the respective pathways.

The air cooler arrangement may include an indirect evaporative cooler“ A second, direct, atiye 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 e9. one or more of a solar collector system, a solar hot water system a heat pump, and an engine jacket coolant, either ly, 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 ant cooling system may include damper arrangements. for selectively . ing 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 . selectivelyIn plural modes with respect to an associated space. fOr example desiccant g, non-desiccant cooling and heating.

Preferably, the ration air does not include any air from the space to which the deh'umidified air is directed. This minimises ductwork, facilitates building al- 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 ration 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. out the method of the invention in an optimal . The control system preferably carries operating mode such as Space indirect or desiccant . g, evaporative cooling. g modes. Preferably the l system comprises at least four s.

As used herein. except where. the context es otherwise. the term "comprise" and variations of the term. such as "comprising". "comprises" and,"ccmprised", are not intended to exclude further additives, ents, integers or steps.

Brief description of the drawings The invention will now be further described by way of e only by reference to the accompanying drawings, in which: Figure 1 is a m 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 fl 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 dIfed 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 precooled. 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. ration of the desiccant wheel 14 is achieved by g hot air '28 in a second ' ~15 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 ration air stream 30 exiting the desiccant wheel in pathway 15. ' Regeneration air 27Is heatedIn a g coil 40 with extemaliy applied heat to obtain hot air 28 for regeneration of the desiccant wheel 14 .20 Desiccant wheel 14 s 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. tion, and second location 14b in which the solid desiccant lies in pathway 15 for the regeneration air 28 to takeup moisture therein as water vapour.

' A single air circulation fan 50 pressurizes fresh ambient air 52 for the process and delivers italong both pathways 11, 15 from a common intake 54 at’the fan. Thus. at a flow divider 56, one fraction 27 of the rised air is ed. along pathway 15 W0 2012/162760 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 12iof 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 dehumidifled and then cooled in turn by indirect evaporative cooler 1_8 and direct evaporative cooler 22.

The re of the air required from "the fan is redUOed, compared with the tional process, through the elimination of the conventional heat recovery heat exchanger. ermore, the pressure drop over the regeneration dair side is' well matched with the re drop over the supply air side, is. the pressure drOps are of similar ude and hence a single fan can provide air at a singlepressure level suitable for both sides of the desiccant process. These s lead to reduced parasitic fan power consumption.

By “similar magnitude” in relation to the re 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 ences in the pressure drop are typically related 'to differences inthe 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 ed 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 es 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 ch es a new control system which preferably ses 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.

Fourswitching 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 15, while damper 64 controls diversion of heated air, downstream of heater 40, as space- heating air to ed space 26. g and opening .of dampers 61-4 is managed by a controller 80, "which is red or programmed to allow selection of various damper position ations to set desired operating modes including desiccant cooling, non-desiccant cooling (in this mse 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. wo 2012/162760 Table 1 Damper Damper Damper position 61 position 62 position 63 position 64 .

The logic that determines the choice of optimum operation mode from data inputs is illdstrated as a flowchart in Figure 3.

The outdoor ambient ve humidity signal can- be directly measured and supplied to the controller. A threshold outdoor relative humidity. below which there is limited advantage in using ant cooling (compared with indirect evaporative cooling), is around 50%.

It is also'possible to use a number of alternative measured signals which ctly infer the r relative humidity and hence provide an approximate tute. For example a time clock can be used to' infer typical approximate l 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 ty signal.

An exemplary ingprofile 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 temperattrre 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 ature is high, but the relative humidity is low. As a result, the system operates predominantly inindirect evaporative cooling mode, even though the hot water temperature is hot enough for desiccant cooling. ' ong hour by hour TRNSYS simulations suggest that a solar desiccant cooling system, based on this , 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 recirculationfair, 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. uction point for this building recirculation air is 3 shown at 7-1 in Figure 2. More generally, the invention envisages that there may well be additional cooling s and/or circuits in the building or in the air ation streams.

It is believed that the inventive configuration, at least in one or more embodiments, is. adaptable as a low-cost t cooling system suitable for residential applications.

Notable advantages includez~ . 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 e 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 (11)

Claims 1.

1. A solid desiccant g system, comprising: means defining a first pathviay for air to be cooled, and a second pathway for regeneration air; structure retaining a mass of solid ant 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 on in which the solid desiccant'lies in said second pathway forsaid regeneration air to take up moisture therein as water vapour; 10 an air heater arrangement in said second pathway upstream of said second location for heating the regeneration air; an air cooler arrangement, ndent 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 15 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 ude.

2. A solid desiccant cooling system according to claim 1 wherein the ng of the. ‘ air delivery device to both of said pathways includes a flow divider at which respective 20 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 ry device is an air circulation fan.

4. A solid desiccant cooling system according to claim 1, 2 or 3 further ing one or more damper arrangements for selectively bypassing the mass of-solid desiccant in 25, the first y 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.

S. 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 n the air cooler .arrangement r includes a second evaporative cooler stage sing a direct 10 evaporative cooler stage. and/or a refrigerative cooling stage, downstream of the indirect evaporative .

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 ration air by “low grade"_heat 15,

9. A solid ant 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 ctly via an ediate heat transfer fluid.

10. A solid desiccant cooling system according to any one of claims 1 to 9 configured 20 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 sclid ant " lies in a flow of air and. dehumidifies that air by adsorption of moisture to the desiccant, 25 and a second Iodation 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 '. W0