Evaluation of a New Solar Air Conditioner
Publication Number: 500-04-062
Publication Date: September 2004
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Executive Summary Introduction
Solar air conditioning has great potential. Sunlight is most plentiful in the summer when cooling loads are highest. For a typical single-story building in California, the daily amount of solar energy available on the roof is roughly 10 times the daily cooling load.
Unfortunately, currently available technologies have not been able to economically use this huge resource. For example, photovoltaic (PV) cells are expensive and require a large portion of the roof to provide sufficient electrical power to drive an air conditioner. The cost for these panels can be ten times that of a conventional air conditioner.
Energy storage is another problem with existing systems. Maximum solar energy generally occurs around solar noon, while the maximum cooling load is several hours later. Storing electricity, such as with batteries, is costly and introduces additional energy losses. Storing cooling in the form of ice or cold water takes a large amount of space in addition to the problems with cost and efficiency. Of course the electrical grid can be used to supplement the energy from the PV panels, but this approach means that the generation and transmission capacity must be available to handle the maximum cooling load, which negates much of the benefits from solar air conditioning.
Thermally driven systems have also seen some use, but have similar problems. These systems use conventional absorption chillers or other cooling equipment that is designed for use with natural gas or steam as an energy source. Unfortunately, the high temperatures required to drive these systems greatly increase the cost of the solar collectors. In addition there is no economical way of storing the thermal energy.New System
The new solar air-conditioning system takes a much different approach. It is a thermally driven system that differs from earlier thermal systems in that it is designed to work with low-temperature solar collectors and includes low-cost energy storage. Energy is stored in the form of a concentrated salt (desiccant) solution that can be used to provide dehumidification. A cooler with a special heat exchanger design combines the dehumidification from the desiccant with cooling from evaporation of water to provide air conditioning.
The preferred salt is calcium chloride. It is inexpensive and is commonly used as road salt. The solar collector can be a shallow pool in a black plastic liner, which allows the sun to evaporate water from the salt solution. The concentrated salt solution is then stored in a tank until it is needed for cooling. If sunlight is not sufficient to concentrate the desiccant, off-peak electricity or natural gas can be used as a back-up heat source. (For a detailed technical description of the solar air conditioner, see pages 6 and 7 in the Introduction.)Project Objectives
The overall objective of the project is to demonstrate a thermally driven solar air conditioner that has the potential of being economically viable compared to conventional electrically driven systems. The immediate objectives were to obtain component test data and modeling results that can be used to support the overall objective.Project Outcomes and Conclusions
There were several important outcomes from this work:
- Low-cost solar collectors can provide effective evaporation of water from the desiccant liquid.
- Several low-cost, high-performance liquid-to-liquid heat exchangers used in the cooler were tested and have promising performance. Leak-proof construction needs further work.
- A 75% reduction in peak electrical demand from air conditioning is feasible based on modeling results.
- Modeling shows that solar air-conditioning systems have the potential to be competitive on a first-cost basis with conventional air conditioners.
This work shows a great potential for solar air conditioners. Further work on the development of the cooler heat-transfer is fundamentally important. The use of low-cost plastic materials that eliminate corrosion problems with conventional metal heat exchangers offers great promise for achieving low-cost, high-performance designs that are essential for creating an economically viable solar air conditioner.
Components for a new, patented, thermally driven, solar air-conditioning system were tested and analysis of the performance of the system was evaluated.
The proposed system uses an inexpensive desiccant, such as calcium chloride, in combination with an indirect evaporative cooler to provide cooling and dehumidification. Calcium chloride absorbs moisture from incoming air. Moisture from the calcium chloride is evaporated in a low-cost solar collector that operates at a very low temperature (typically less than 120°F). The collector can be a simple pool in a black plastic liner that is open to outside air. A transparent cover may be added to prevent accumulation of rainwater. Concentrated calcium chloride solution can be stored for use at night or during cloudy periods when sunlight is not available. Evaporation rates of approximately .5 to 1.0 lbm/ft2/day were demonstrated in a small pool under summer conditions.
The heat-exchange system uses 100% outside air. An indirect evaporative cooler uses exhaust air to cool desiccant liquid, which cools incoming air. Modeling shows a coefficient of performance (COP) of 1.35 assuming no credit for outside air. If ventilation load is included (100% outside air requirement), the COP is approximately twice this value. Projected peak electrical demand (mainly fans and pumps) is about 25% of a high-efficiency base rooftop unit. Estimated factory cost of the system is approximately $430 per ton with an installed cost of less than $2000/ton for rooftop applications. These projected costs should be competitive with conventional rooftop air-conditioners on a first-cost basis.
Table of Contents
Public Benefits to California
Development Stage Assessment
Appendix: US Patent 6,513,339
List of Figures
Average Global Horizontal Radiation for California in August
Schematic Diagram of the Solar Air Conditioner
Evaporation Test Data from Two Open-Pool Solar Collectors
Weather Data During Collector Tests
First Liquid Heat Exchanger Geometry
Second Liquid Heat Exchanger
Comparison Between Curve-Fit Effectiveness and Manufacturer’s Data for an Evaporative Pad
Comparison Between Curve Fit and Manufacturer’s Pressure Drop Data for an Evaporative Pad
Modeled Cooler Temperatures
Effect of Outside Air Requirement on Rated Capacity for Typical California Design Conditions
Development Assessment Matrix
List of Tables
Cooler Model Results
Projected Costs for New Solar Air Conditioner