Evaluation of New Solar Air-Conditioning System

This project was funded under Energy Commission's PIER Energy Innovations Small Grant Program (EISG), which supports the development of new technologies that are in the proof-of-concept stage.

EISG Grant Number:  00-16

Organization:  WorkSmart Energy Enterprises, Inc.

EISG Status:  Completed 2005

Contact: David Michel III, Energy Systems Research Office, Energy Innovations Small Grant Program (916) 651-9381

Project Description:

Air conditioning powered by solar energy has great potential, in part because high demand for cooling usually coincides with plentiful sunlight. The intensity of solar energy on the roof of a typical single-story building in California is roughly ten times the cooling requirement for the same building.  Solar air conditioning at an economically competitive level could reduce electricity costs for residential and small commercial customers. This would cut the growth of peak electric demand and ease the increasing pressures on generating capacity, transmission, and distribution.  Currently available technologies are neither practical nor cost-effective.  Photovoltaic (PV) systems require a large roof area and cost many times more than a conventional air conditioner.  Thermally driven absorption cooling requires costly, high-temperature collectors and undesirable cooling towers. Furthermore, these systems have a disconnect of several hours between peak cooling capacity and peak cooling demand. That in turn requires electric or thermal storage in order to maximize the solar contribution. 

A solar air-conditioning system employing relatively inexpensive low-temperature collectors, coupled with an innovative desiccant dehumidification and evaporative process, provides a new prospect for cost-effective solar cooling.   If proven practical and economical, the savings potential in the California market for rooftop air conditioning would be 8.5 billion kilowatt hours (kWh) in energy and $1 billion per year. This corresponds to a reduction in electric demand in California of 5,500 megawatts (MW).   The potential nitrogen oxides (NO_x) reduction in California is 2,400 tons annually, based on the state average 2000 annual NO_x output emission rate of 0.564 pounds per megawatt hour (lb/MWh).

The research concept couples modest-cost, low-temperature collectors with a low-cost calcium chloride solution for desiccant dehumidification and thermal storage. The addition of an evaporative cooler produces air conditioning at a competitive cost. The economic viability of this concept depends on optimizing the system and its components and on developing two key innovations—a low-cost heat exchanger and a solar thermal desiccant-regeneration subsystem. 

A calcium chloride solution concentrates (regenerates) while passing over the solar collector array.  The concentrated calcium chloride solution cools via a plastic liquid-to-liquid heat exchanger and is then exposed to incoming outside air through a direct-contact enthalpy exchanger, similar to an evaporative cooler.  Exhaust air from the conditioned space cools evaporatively, and the cooled water from the evaporative cooler sump reduces the temperature of the warm, concentrated calcium chloride solution through the plastic heat exchanger.

Proposed Outcomes:

The goal of this project was to determine the feasibility of a thermally driven, solar air-conditioning system employing desiccant dehumidification low-temperature collectors for desiccant regeneration and evaporative cooling.  The researchers established the following project objectives:

1. Identify a design for a leak-free plastic liquid-to-liquid heat exchanger with a heat-transfer coefficient of 50 British thermal units per hour per square foot per degrees Fahrenheit (BTU/hr/ft²/ °F).
2. Achieve collector water-evaporation rate of 1.0 pounds per day per square foot (lb/day/ft²).
3. Plan for the first cost in the same range as high-efficiency electric air conditioning evaporator rooftop systems ($1,800 to $2,200 per ton installed).

1. Nine different designs of low-cost, liquid-to-liquid heat exchangers were evaluated; two were tested.  The preferred configuration consisted of plastic sheets welded together to form two counter-flow channels. It was the easiest to assemble and achieved a heat-transfer coefficient of 40 BTU/hr/ft²/ °F, but the development of small circuit-to-circuit leaks prevented lengthy and repeatable testing.
2. Sample solar-collector tests measured the evaporation rate from a calcium chloride solution.  The sample collector consisted of a plastic plate filled with calcium chloride solution and covered with a black polyethylene film.  The measured evaporation rates ranged between 0.5 and 1.0 lbm/day/ft² under partly cloudy summer conditions in Northern Virginia.
3. The estimated cost per ton was based on modeled subsystem sizes and projected costs for materials, factory labor, mark-up, freight, and installation.  The total projected price to an end user was $1,825 per ton.  Electricity requirements are expected to be on the order of 0.25 kilowatts per ton (kW/ton), about ¼ that of a high-efficiency electric rooftop package.  Annual water usage is estimated at 6,000 gallons per ton for a typical California application. 

• Low-temperature solar collectors can provide effective regeneration of a calcium chloride liquid desiccant solution.  Average evaporation rates of 0.5 to 1.0 lbm/ft²/day are achievable in California with simple open-collector designs.
• A low-cost, high-performance, liquid-to-liquid heat exchanger was tested with an overall heat-transfer coefficient in the range of 40 BTU/hr/ft²/ °F.  Leak-proof construction and longevity are important areas for future attention.
• A projected 75% reduction in electricity use for air conditioning corresponds to an electric Coefficient of Performance (COP) equivalent of 14.  By contrast, a high-efficiency rooftop package has a COP of 3.5.
• Water requirements appear to be modest and do not add appreciably to the operating expenses.  At $3/1,000 gallons, the cost for water is $18/ton/yr, a fraction of the cost for an electric air conditioning evaporator unit.  If the entire California inventory of rooftop air conditioning switched to this approach, the annual water requirements would be 100,000 acre-feet.  In contrast, California consumes 9.5 million acre-feet annually for urban uses.
• Modeling, coupled with preliminary cost estimates for materials, labor, markups, and installation, indicates that the solar air-conditioning system has the potential to achieve installed costs of $2,000/ton, on par with the typical installed cost of efficient rooftop models.

The technical feasibility of a novel solar air conditioner incorporating low-cost materials has been proved.   Simple heat and mass-transfer tests were performed with representative material samples.  The measured properties were used to size and cost the system.  Beyond the scope of this small grant, considerable work remains to scale up the subsystems; to test prototype systems for performance and durability in an outdoor environment; and to confirm cost estimates for manufacturing, distribution, and installation.

Benefits to California:

The primary benefit to the ratepayer from this research is increased affordability of electricity in California.  The novel solar-air-conditioning concept would reduce the biggest cause of peak electricity demand. That would enable increased utilization of the generation, transmission, and distribution system and would delay new generating and transmission investments, lowering the cost of delivered electricity.  Reducing peak demand also helps relieve congestion and improves the reliability of the power supply. 

An economic solar air conditioner would also help California adopters of the technology control their energy expenses. The light commercial and small industrial sectors would best be able to utilize this technology.  The electricity usage for rooftop air conditioning in California is 11.4 billion kWh per year.  With energy savings of 75% projected, the displacement potential of solar air conditioning in these California sectors is estimated at 8.5 billion kWh annually. That corresponds to a demand reduction in the vicinity of 5.5 GW and a consumer cost savings of $1 billion per year.

After taking into consideration (a) research findings in the grant project, (b) overall development status, and (c) relevance of the technology to California and the PIER program, the Program Administrator has determined that the proposed technology should be considered for follow-on funding within the PIER program. 

Receiving follow-on funding ultimately depends upon (a) availability of funds, (b) submission of a proposal in response to an invitation or solicitation, and (c) successful evaluation of the proposal.

The solar air-conditioning concept is a novel approach that recognizes the importance of initial cost to economic viability and market acceptance.  Although the scale-up of the concept, its durability, and its true cost remain uncertain, it merits funding for the next development step.  Further work should address the following:

• Heat-exchanger design and fabrication techniques for low cost and high performance.
• Heat-exchanger material that is inexpensive yet durable for ten to fifteen years of operation.
• Collector subsystem design that meets the $70/ton material cost target yet is rugged enough to endure ten years of outdoor operation.
• Bench-scale system test to verify cooling capacity and parasitic electricity requirements.
• Following additional research and laboratory prototype testing, verification of the $2,000-per-ton installed target requires in-depth analysis of material, manufacturing, distribution, sales, and installation costs.

This project is part of the research portfolio of the California Energy Commission. The Energy Commission supports energy research and development that improves the quality of life in California by bringing environmentally sound, safe, reliable, and affordable energy services and products to the marketplace.