Q&A
Solar-Assisted Air Conditioning: What Engineers Need to Know
From ASHRAE Journal Newsletter, September 8, 2020
Solar-assisted air-conditioning systems are part of the HVAC&R industry’s solution to develop low-energy, low-emission systems. But some solar-assisted AC systems may work better than others.
Earlier this year, the Florida Solar Energy Center at the University of Central Florida released a report1 detailing its side-by-side testing of conventional heat pump systems with and without a solar thermal collector. The results showed the solar thermal collector did not increase the system’s performance in air-conditioning mode. Tim Merrigan, Life Member ASHRAE, member of ASHRAE Technical Committee 6.7, Solar and Other Renewable Energies, explains what engineers need to know about the various types of solar-assisted cooling systems and how they work.
1. How does solar-assisted air conditioning work?
Solar-assisted air-conditioning (AC) is found in four main configurations, and they all work somewhat differently:
- Absorption chillers;2
- Adsorption chillers;3
- Desiccant-enhanced AC;4 and
- Photovoltaic (PV) powered vapor compression AC.
The first three require heat input to either regenerate absorption refrigerant pairs, desorb vapor from an adsorbent or drive water from a desiccant, so solar thermal systems can be used to provide the required heat in these configurations. Both absorption and adsorption chillers provide sensible and latent cooling, while desiccant systems provide latent cooling only. Liquid and solid desiccant systems are also the simplest solar-assisted AC, since the desiccant removes moisture from the air and then is regenerated from the solar heat.
For vapor compression AC, which also provides both sensible and latent cooling, only solar electric systems are suitable to provide a solar assist. Adding solar heat to a refrigerant in a vapor compression cycle can only assist a heat pump in heating mode. Adding heat to the refrigerant does not work in air-conditioning mode.5
2. What are the benefits of using solar-assisted air-conditioning systems?
Solar-assisted air conditioning is also obviously addressing the enormous growth in air conditioning and cooling worldwide. By using renewable energy, solar-assisted AC systems are decreasing the use of fossil fuels and reducing annual energy costs. In addition, solar cooling systems use natural refrigerants, such as lithium chloride, water and zeolites.6 So solar cooling is addressing the critical issues of greenhouse gas emissions and ozone depletion.7
3. Are there particular building types or projects or climate zones where solar-assisted air-conditioning systems work best?
While all solar-assisted AC systems work on buildings with cooling loads, buildings with cooling loads that are simultaneous with peak summer solar radiation are ideal. For example, if a school is not occupied in the summer, it typically would not be a cost-effective candidate for a solar-assisted AC system.
While all solar-assisted AC systems work well in sunny climates, desiccant-enhanced solar-assisted AC systems work best in humid climates, where latent cooling loads are significant.
4. What are the challenges the solar cooling market is facing, and how do those challenges affect innovation/technologies?
While the market for solar cooling has historically been small (due primarily to economics8), there has recently been more interest in Europe, especially in Spain and Italy.9 Small modular adsorption cooling systems that can be powered with solar thermal energy are being produced by companies in Italy and Germany.10
As more large and small solar cooling systems come online, we can expect to see more long-term performance data on the systems, which will help convince the market of their feasibility.11 In addition, the performance of large absorption chillers has been slowly improving. Correspondingly, with variable speed motors, the power needed to run pumps and cooling towers has decreased, increasing overall efficiency.6
5. What are some resources designers should be using to correctly use or identify proper solar-assisted air-conditioning systems?
ASHRAE Handbook includes several resources:
- 2017 ASHRAE Handbook—Fundamentals, Chapter 2: “Thermodynamics and Refrigeration Cycles”;
- 2018 ASHRAE Handbook—Refrigeration, Chapter 18: “Absorption Equipment”;
- 2019 ASHRAE Handbook—HVAC Applications, Chapter 36: “Solar Energy Use”; and
- 2020 ASHRAE Handbook—HVAC Systems and Equipment, Chapter 24: “Desiccant Dehumidification and Pressure-Drying Equipment.”
6. What are some common misunderstandings about designing these systems?
In the warmer parts of the U.S. where air conditioning is prevalent–such as Arizona and Florida, but even in other southeastern and western states–some companies are promoting, including at industry trade shows, solar-assisted air-conditioning systems that add solar heat to a vapor compression AC system. The marketing materials typically show a schematic or flowchart of a typical vapor compression air-conditioning cycle–evaporator, compressor, condenser and expansion device. They also include a solar thermal collector, either evacuated tube, concentrating or flat plate, in-between the compressor and the condenser.
Typical “hybrid solar cooling” marketing materials claim that the solar thermal collector heats the refrigerant in the air-conditioning cycle and helps the compressor do its work, increasing energy efficiency and energy savings to the customer.
But, in fact, according to the principles of thermodynamics, the solar thermal collector actually increases the superheat of the refrigerant leaving the compressor during the daytime, and, therefore, lowers the effectiveness of the condenser because that superheat now has to be removed before the refrigerant can begin to condense. At night, the solar collector could possibly act as a desuperheater and take away heat from the refrigerant, but only if an additional heat exchange circuit is set up through the now “non-solar” collector when the refrigerant is flowing.
Based on recent testing in Florida,1 the overall 24-hour operation of such a “hybrid solar cooling” system results in a decrease to the energy performance of the air conditioner, which obviously does not provide any energy savings. In fact, it would actually cost more to operate an air conditioner in the manner described by some “hybrid solar cooling” marketing materials. Quoting from the conclusions of the 2020 FSEC Energy Research Center test report on one “hybrid solar cooling” system, “…the technology was not shown to be viable or of any benefit.”1
7. Is there anything else you think design engineers need to know about this topic?
Design engineers should know to ask for the AHRI Standard 210/24012 test results for any air-source heat pumps and air conditioners less than 65,000 Btu/h (19 kW).
A number of solar thermal-based absorption, adsorption and desiccant “solar cooling” systems as well as solar electric-based “solar air-conditioning” systems use photovoltaic (PV) modules to supply electricity to the compressor and outdoor condenser fan unit. These systems do not violate the principles of thermodynamics and do provide energy savings. Residential and commercial customers can benefit from their adoption and use.
References
1. Sherwin, J., P. Fairey. 2020. “Side-by-Side Testing of SolAire Solar AC,” FSEC-CR-2100-20. FSEC Energy Research Center. https://tinyurl.com/yc64ov9a.
2. 2019 ASHRAE Handbook—HVAC Applications, Chap. 36, “Solar Energy Use,” pp. 36.18–19 (“Solar Cooling with Absorption Refrigeration”).
3. Wang, K. E. Vineyard. 2011. “Adsorption refrigeration: new opportunities for solar.” ASHRAE Journal 53(9):14–24.
4. 2020 ASHRAE Handbook—HVAC Systems and Equipment, Chap. 24, “Desiccant Dehumidification and Pressure-Drying Equipment.”
5. ASHRAE. 2020. "Solar Assisted Air Conditioning: How Solar Energy Can Cool Down Your Building,” Seminar 5, 2020 ASHRAE Winter Conference. https://tinyurl.com/ya5wa9xu.
6. Solar Heating & Cooling Programme. 2019. “Solar Heat Worldwide 2019, Chap. 4.5. https://tinyurl.com/yamgxs3h.
7. McGowan, M. 2019. “Breaking down barriers to expand solar cooling applications.” ASHRAE Journal 61(8):42–44.
8. solarthermalworld.org. 2020. “Solar cooling: insights into a niche market.” https://tinyurl.com/yakwvf9b.
9. solarthermalworld.org. 2019. “Solar cooling at Intersolar 2019.” https://tinyurl.com/y9agu9h5.
10. solarthermalworld.org. 2019. “Attractive solar cooling market in Italy.” https://tinyurl.com/y7c2tc8q.
11. Solar Heating & Cooling Programme. 2020. “Project (Task) Publications.” https://tinyurl.com/y9xvuzkk.
12. AHRI Standard 210/240-2017, Performance Rating of Unitary Air-conditioning and Air-source Heat Pump Equipment.