Hybrid Solar Desiccant Cooling System , Seminar Reports | PPT | PDF | DOC | Presentation |


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.

Low temperature heating dominates all residential, commercial, and industrial end uses of energy within buildings. Nearly 61% of the energy used across all sectors of the economy is for low temperature heating uses. These heating end uses include space heating, industrial process heating, water heating, boiler heating, and clothes drying. The second greatest energy end use is for cooling. Another 13% of all building energy end use is for refrigeration or space cooling. In the late 1990's, the combination of all heating and cooling energy end uses cost US consumers nearly $180 billion per year.

Most of this energy use requires the consumption of fossil fuels. In most cases, the energy conversion devices, such as boilers or electric heat pumps, operate at low efficiency compared to the fuel they consume. In almost all fossil energy heating and cooling systems, the conversion from the primary fuel (gas, oil, coal, etc.) to heating or cooling is done at less than 100% efficiency. This is often described as a coefficient of performance of less than 1.0. (COP < 1).However, there are three technologies that operate at what can be called super efficiencies. These technologies convert primary energy into heating or cooling capacity with COP’s between 2 and 5.

These three technologies include solar thermal heating, evaporative cooling, and desiccant drying. By using a combination of simple, low energy, physical phenomena, and widely distributed low cost energy and water resources, these three technologies are recognized as super efficient at delivering heating and cooling.

 Solar air and water heating system have been shown to have a COP of 4 or greater at providing solar heat with little expense in fan or pumping primary energy. Evaporative cooling systems have been demonstrated to operate at a COP of 5 when used in dry conditions. Desiccant system with evaporative final cooling can operate at a COP of more than 2.

However, the application of these technologies has been restricted by their individual limits in responding to:

 1) High temperature or

 2) High levels of humidity, or

3) Harsh economic realities caused by seasonal idleness of expensive heating and cooling equipment.

Evaporative cooling is only effective for comfortable cooling in dry climates. When outdoor humidity rises, the cooling capability of direct evaporative systems declines unless occupants are willing to suffer with high humidity. Even in climates that suffer only a few weeks of high humidity, most consumers will select low efficiency compression refrigeration cooling systems for comfort cooling. Since most customers buy only one system, the low COP compression systems will be the only cooling systems installed. Thus, the rest of the year’s super efficient evaporative cooling capacity is lost for lack of a few weeks of dry air.

 Solar space heating suffers a similar fate due to the typical high cost of flat plate collector systems and the lack of useful energy cost savings delivered in the summer months. The seasonal decline in cost savings reduces the overall cost savings the systems can deliver in any given year. This stretches out payback periods for traditional flat plate systems and makes them uneconomic for most space heating applications.

Additional research is recommended in the following areas:

1) Evaluate alternative solar tile roof components to establish the most cost effective summer outlet temperature for the solar roof when supporting desiccant regeneration.

2) Prepare a desiccant wheel that is optimized to the outlet temperatures of the solar roof.

3) Assemble the components in an operational prototype and test for cooling performance in a suburban setting.