Thermodynamic Solar Panels

Key Takeaway: Thermodynamic solar panels are clever in theory, but disappointing in practice. Pairing PV solar panels with a power diverter is a much better alternative.

What are Thermodynamic Solar Panels?

Thermodynamic solar panels (AKA solar-assisted heat pumps) extract heat from daylight and outdoor air to give you hot water. Their main advantage is better efficiency than an immersion heater. However, thermodynamic solar panels compare badly to the alternatives. Serviceability is a particular problem. So don’t buy thermodynamic solar panels before you read this article (or ever, preferably)!

How do Thermodynamic Solar Panels Work?

Thermodynamic solar panels work on the same principal as a fridge: Pumping heat from one place to another. The difference is that instead of cooling down your food they heat up your water. This makes them a kind of hybrid between a solar thermal panel and a heat pump. If you like, see the green box for more details. Feel free to skip the technical explanation if that’s not for you 🙂 From the user’s point of view, thermodynamic solar panels do basically the same thing as an immersion heater – but use less electricity.

The diagrams shows the components of a thermodynamic solar panel system connected in a loop in the following order:  1)  Expansion valve; 2) Thermodynamic solar panel;  3)  Compressor;  4)  Heat Exchange Coil.  The heat exchange coils is installed inside a hot water cylinder.
Thermodynamic solar panel schematic. Image License: CC-BY. Credit:

Technical Explanation: First, fluid passes through an expansion valve and becomes very cold. To make it less cold, it then flows into the thermodynamic solar panel. Once inside the panel, the fluid sucks up any warmth it can find. It will suck warmth out of sunlight, rain, and even the ambient air. Next, the fluid goes into a compressor. Compressing the fluid makes it hot. This compression and heating is the key step. Once the fluid is hot, it flows through a coil inside your hot water cylinder. From here, the heat goes into the contents of your thank, giving you hot water. The process is now complete and the fluid goes through the expansion valve again to continue the cycle.


  • More efficient than immersion heaters
  • Light weight makes installation easy
  • Takes up relatively little roof space
  • Does not lose efficiency over time
  • Antifreeze not required
  • Can work at temperatures well below zero
  • Can work without sun and even at night


  • Uses much more electricity than solar thermal panels
  • Not optimised to take advantage of cheaper night-rate electricity
  • Less efficient than air-source and ground-source heat pumps
  • Don’t provide hot water for space heating
  • Reliability problems
  • Difficult to service if something goes wrong

Cost to Run Thermodynamic Solar Panels Versus Immersion Heaters

In 2014, a company called Narec tested the efficiency of a thermodynamic solar panel system. They found that the coefficient of performance for the system was between about 1.5 and 2.2. This means that the system used approximately 33-55% less electricity than a typical immersion heater would have. In euro terms, that’s about €250 – €450 per annum for a family of five. So thermodynamic solar panels beat immersion heaters in terms of efficiency and running costs.

Cost to Run Thermodynamic Solar Panels Versus Heat Pumps

Thermodynamic solar panels look a lot less favourable when you compare them to heat pumps. First of all, heat pumps tend to have a better coefficient of performance (~2 – 3.5) than thermodynamic solar panels (~1.5 – 2.2). Secondly, many heat pumps provide not only hot water to your taps, but also hot water for space heating. Overall, a heat pump saves more energy than a thermodynamic solar panel.

An immersion heater provides one unit of heat energy for every unit of electricity consumed.A thermodynamic solar panel provides about two units of heat energy for every unit of electricity consumed.A heat pump provides about three units of heat energy for every unit of electricity consumed.
Thermodynamic solar panels are more efficient than immersion heaters, but not as efficient as heat pumps. Image License: CC-BY. Credit:

Solar Panel + Heat Pump 2-in-1

In theory, thermodynamic solar panels combine the advantages of both solar thermal and heat pumps in one device. However, the reality is more of a “worst of both worlds” result. One the one hand, thermodynamic solar panels lack the insulating glass covers of solar thermal panels. For this reason, the fluid in the panels struggles to reach the high temperatures that solar thermal can achieve. On the other hand, thermodynamic solar panels lack the high-power fan of an air-source heat pump. This hinders their performance whenever the sun isn’t out.

Are They Eco-Friendly?

Thermodynamic solar panels use less electricity than immersion heaters. This is a clear positive for the environment. However, heat pumps are a better choice because they use even less electricity. Solar thermal panels are betters still, as they use only a tiny amount of electricity for the pump. Best of all are solar PV panels with can provide you with both hot water and electricity. Overall, thermodynamic solar panels compare well to immersion heaters but poorly to other options in terms of energy consumption.

Another consideration is the fluid inside the panels. Both thermodynamic solar panels and heat pumps depend on refrigerants to make them work. Often, these refrigerants contain HFCs. This is an issue because HFCs are extremely potent greenhouse gases. Any leaks pose a serious environmental risk. On the other hand, immersion heaters and solar thermal panels don’t contain refrigerants. If you’re concerned about the environmental impact of HFCs then look for heat pumps and thermodynamic solar panels with supercritical CO2 or hydrocarbon refrigerants, instead of HFCs.

Thermodynamic:  Works day and night, moderate running cost, high initial cost, risk of HFC leaks.Solar thermal:  Does not work at night, low running cost, moderate initial cost, no HFCs.Heat pump:  Works day and night, low-moderate running cost, high initial cost, typically uses HFCsImmersion heater:  Works day and night, high running cost, low initial cost, no HFCs.
Some of the main pros and cons of thermodynamic solar panels versus alternative options. Image License: CC-BY. Credit:

Elephant in the Room: Reliability and Servicing Issues

Unfortunately, thermodynamic solar panels have a poor track record for reliability and can be difficult to get serviced. I know this because I regularly get emails from Irish homeowners asking if I know anyone who can fix broken thermodynamic solar panels. Generally, the homeowners have been unable to get any assistance from the original installer. Many seem to be seriously struggling to get their panels fixed. And worst of all, I don’t know of anywhere to send them for help. This is clearly a situation that you want to avoid.

Reliability and serviceability are the top reasons why I recommend solar PV over solar thermodynamic every time.

Hot Take on Thermodynamic Solar Panels

Thermodynamic solar panels are a clever in theory, but disappointing in practice.

PV Solar Panels For Hot Water

If you want reliable hot water from the sun, then consider using PV solar panels in combination with a power diverter. Not only is this system more reliable and serviceable than thermodynamic solar panels; it also gives you greater savings on your energy bills. This is because PV solar panels produce electricity which, in addition to heating water, can be used to power your whole home. Overall, it’s possible to reduce your electricity bills by 50% or more with solar PV.

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