Hybrid solar Desiccant Cooling System

Definition:

Using excess summer heat from solar collectors to drive desiccant cooling systems is often proposed. A two wheel desiccant system using solar heat for desiccant regeneration is typically discussed. The two wheel system uses a desiccant wheel that is “matched” with a heat exchanger wheel. The heat exchanger recycles heat for the desiccant regeneration and improves system efficiency. These systems are generally limited to delivering warm dry air or cool humid air in most parts of the US. A newly patented desiccant cooling cycle creates two dry air streams. This new cycle uses indirect evaporative cooling of one air stream to cool the second stream. Additional direct evaporative cooling allows cool and dry air to be delivered to the building. Regeneration exhaust heat can provide water heating. Combining the system with a new solar air heating system should provide a significant solar heating, cooling, and hot water delivery system.

A desiccant material naturally attracts moisture from gases and liquid. The material becomes as moisture is absorbed or collected on the surface; but when heated, the desiccant dries out-or-regenerate and can be use again. Conventional solid desiccant include silica gel, activated alumina, lithium chlorate salt and molecular sieves. Titanium Silicate a class of material called 1m, and synthetic polymer are new solid desiccant material design to be more effective for cooling application. Liquid desiccant include lithium chlorate, lithium bromide, calcium chloride and triethylene glycol solution.

In a dehumidifier, the desiccant removes moisture from the air, which release heat and rises the air temperature. The air is then cooled by heat re-covers units and cooling devices such as evaporative cooler or the cooling coil of a conventional air conditional. In a stand alone desiccant system, air is first dried, and then cooled by a heat exchanger and a set of evaporative coolers. This system is free of ozone-depleting CFC and HCFC refrigerant. In most systems, a wheel containing desiccant continuously dehumidify outside air entering the cooling unit. The desiccant is then regenerated by thermal energy.

Desiccant Evaporative Cooling System:

A typical desiccant cycle can be cost effective when removing humidity from the air. However, regeneration of the desiccant requires heating roughly equal to the energy it provides for dehumidification. When using evaporative final cooling, the system can deliver a range of warm dry air or cool humid air at relatively high COP.

A typical two wheel desiccant cycle is shown in Figure 2. The psychometrics for the cycle is shown in Figure 3 along the lines from A to B to C and the “2 wheel limit line”. The regeneration cycle is shown along E, G, I and J. The “2 wheel limit line” in Figure 3 represents the continuum of temperature and humidity possible by evaporative cooling the dry air from point C to D. As shown, the line does not deliver both cooler and drier air than the original state point E. To achieve the necessary cooling that removes both internal and external heat gain and humidity loads, the condition along the line C to D must be substantially cooler and drier than the existing state point E within the building.

The line C’ D represents one such cooler drier condition. To achieve this condition usually requires an additional cooling system that completes the final cooling from point C to point C’. Compression refrigeration is most often used for this final cooling in conjunction with a desiccant system for dehumidification. However, in most cases, consumers will buy only one cooling system, a compression system, to meet their entire cooling needs.

New Technologies

Recent patents in each technology have overcome some of the problems holding back greater deployment. Tests of these new technologies in the past 4 years indicate that workable systems can be deployed. These systems have shown the technical capacity to deliver solar heating and desiccant cooling with indirect and direct evaporative cooling. Specifically, these new technologies include the solar thermal tile system shown in Figures 4 and 5 and the NovelAire desiccant evaporative cooling cycle shown schematically, in Figure 6.

The solar thermal tile system is a mid temperature air heating collector. It is designed to function as the weather tight roof of a building or as a rack mounted solar collector on low sloped roofs or in ground mounted applications. It is designed to be installed at a cost comparable to high quality slate and tile roofing, which is substantially less expensive than existing mid temperature collectors. As a result, the system can be economically installed to handle the larger space heating loads, even with the seasonal reduction in productivity during the summer months.

Stagnation tests show that the systems can achieve internal air temperatures of greater than 200 degrees F (94 C) and more than 130F (72 C) above ambient temperature. An air flow test with an early prototype showed outlet air temperatures of 160 -180 F (71-82 C) are possible. Higher temperatures are expected with optimal orientation, improved materials such as selective surface absorbers, and optimal air flow.

The system is of sufficiently low cost to deploy a large area to deliver a large volume of air for winter space heating, and deliver high air outlet temperatures particularly in the summer. This provides an opportunity to support desiccant regeneration with the large quantities of excess summer heat. Because the system is an air heating system, it is well suited for direct delivery of solar heated air for desiccant regeneration

Advantages

(1) Desiccant evaporative cooling, used as stand-alone system or to supplement conventional cooling equipment, removes moisture from the air without the use of ozone-depleting compounds.

(2) Micro organism are well protected indoors by the moisture surrounding them if humidity is above 70% they can cause acute diseases and cause the building structure and it’s contain to deteriorate.

(3) Direct indirect and evaporative cooling system is less expensive than vapour compression system.

(4) Hybrid system can provide year round comfort.

(5) It decreases the electrical demand.

(6) Desiccant based system can reduce moisture much below 40f dew point temperature, while the conventional cooling can only dehumidify the air to temperature above 40f dew point temperature.

(7) Desiccant system can often permit reduction in size of the conventional system (vapour compression system), because part of cooling load (dehumidification load) is shifted to desiccant system. Size reduction not only save the energy, but it also decreases the electrical demand and may reduce initial capital investment.

Reference

1) P. L. Dhar, S. C Kaushik, sanjeev Jain, Thermodynamics analysis of desiccant augmented evaporative cooling cycle for Indian condition, ASHRAE TRANSACTIONS, 1995.
2) William a Blending, Marc P. F. Delmas, novel desiccant cooling system using indirect evaporative cooler, ASHRAE TRANSACTION, 1997.
3) U.S. Patent 5,651,226 to Archibald dated July 29, 1997.
4) U.S. Patent 5,758,508 to Belding, et.al. dated June 2,1998.