Solar-assisted heat pump

What are solar-assisted heat pumps, and how do you compare them?

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According to the U.S. Energy Information Administration, space heating and water heating can account for almost two thirds of energy use in U.S. homes—those bills definitely add-up! You can use many different types of energy efficient heating systems to offset these costs, including solar-assisted heat pumps (SAHPs), which some manufacturers claim can have payback periods as low as two to three years. These systems combine technology similar to solar hot water and air source heat pumps in order to heat water or small spaces in your home. SAHPs have existed since the 1970s, but have recently started gaining more attention due to their high efficiency.


An overview of solar-assisted heat pumps


  • SAHPs combine thermal solar panels and heat-pumps to produce heat
  • The efficiency of a SAHP varies based on its configuration and its surrounding environmental conditions
  • SAHPs can include different types of solar collectors
  • Coefficient of performance is the primary way to measure the efficiency of SAHPs

How does a solar-assisted heat pump work?

SAHPs use thermal energy from the sun and heat pumps to produce heat. While you can configure these systems in many different ways, they always include five main components: collectors, an evaporator, a compressor, a thermal expansion valve, and a storage heat exchanging tank.

Collectors

You may be familiar with photovoltaic (PV) solar panels, which convert energy from the sun into electricity – but have you heard of collectors, or thermal solar panels? Instead of producing electricity, collectors convert the sunlight into heat through their absorber plates. The heat generated is transferred to the refrigerant, a substance that absorbs and expels heat throughout the system. 

There are various types of collectors you can use to maximize the efficiency of your SAHP, depending on surrounding environmental conditions:

Collectors

Flat plate collectors

Flat plate collectors contain large, flat absorbing plates that transfer heat to the refrigerant within the collector. They operate at maximum efficiency when the sun is directly overhead so they are best for areas with a lot of sunlight.

Evacuated tubes 

Evacuated tubes include rows of parallel transparent glass tubes that are connected to a heater pipe containing the refrigerant. They operate at a higher efficiency than flat plates, but can also be prone to overheating and cracking in hot temperatures, making them more difficult to maintain in hot climates.

PV-T or hybrid

PV-T or hybrid collectors combine PV solar cells and thermal panels. The excess heat produced by the PV cells is transferred through the thermal panel to the refrigerant. They significantly improve the efficiency and performance of SAHPs, especially since you can use electricity from the PV to power the compressor. They don’t tend to overheat and can work well in both warm and cold climates.

Thermodynamic

Thermodynamic collectors use heat from ambient air and solar to heat the refrigerant as it passes through the panel. They are not solely dependent on sun exposure, and work at night and in the winter. You can install thermodynamic collectors on the sides of buildings as well as on roofs.

Evaporator

After the collectors heat the refrigerant, the fluid evaporates into a gas. In direct expansion SAHPs, the refrigerant is circulated directly through the solar collectors and the absorber acts as the evaporator. In indirect expansion SAHPs, the refrigerant is part of a closed loop system in which it passes from the collector to a heat exchanger that serves as the evaporator.

Evaporators

Thermal exchange valve

The thermal exchange valve increases the efficiency of the SAHP by regulating the rate at which the refrigerant flows into the evaporator to maximize energy output.

Compressor

The gaseous refrigerant moves through a compressor, which pressurizes it and concentrates the heat. The compressor requires electricity to run, which can come from fossil fuels or renewable energy sources, such as PV solar panels.

Storage heat exchanging tank

The pressurized refrigerant passes through a series of pipes known as heat-exchangers, or condensers. The refrigerant condenses into a liquid and the system transfers the produced heat from the pipes into the water in your storage tank. Your water is now hot and you’re set to take that hot shower!

Questions to ask when comparing solar-assisted heat pump options


The performance of your SAHP will vary based on how it’s configured and where you live. You should get answers to the following questions before installing a system:

  • Will you use your solar-assisted heat pump to heat your water, space, or both?
  • What type of climate do you live in and what system will work best for that climate?
  • What type of energy do you plan to use to power the compressor?
  • Do you already have components you intend to integrate with your solar-assisted heat pump?

How to evaluate the efficiency of solar-assisted heat pumps

Before choosing a SAHP, you should compare the Coefficient of Performance (COP) of various systems. COP is a measure of the efficiency of the heat pump based on the ratio of useful heat produced compared to its energy input. Higher COPs equate to more efficient SAHPs and lower operating costs. While the highest COP that any heat pump can achieve is 4.5, heat pumps with COPs above 3.0 are considered highly efficient.

Power your compressor with PV solar panels

To maximize your monthly savings, you can install PV solar panels in addition to your SAHP to power your compressor using free, clean electricity. By joining the EnergySage Marketplace, you can receive up to seven solar quotes from installers local to your area. Just make sure to note in your account if you plan on installing a SAHP so installers can quote a solar system size to meet your anticipated electricity needs. 


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About Emily Walker

Emily is the Content Manager & Research Analyst at EnergySage, where she enjoys making energy fun and easy to learn about! She has a background in environmental consulting and has degrees in Environmental Science and Biology from Colby College. Outside of work, Emily is pursuing a Master of Science from Johns Hopkins University in Environmental Science and Policy. She also loves hiking, tending to her collection of houseplants, and trying out new restaurants and breweries whenever possible.

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