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An estimation of comparative running costs of Solar Thermodynamic Domestic Hot Water system

A comparison of running costs of thermodynamic solar hot water and fossil fuels

How the Energie Solar Thermodynamic system Works

At Cernunnos-Homes, having researched the market thoroughly, we believe that by far the best year round domestic solar system is the Energie-Solar Thermodynamic system.

The Energie-Solar system is the first “next-generation” solar thermal (thermodynamic) system that uses a refrigerant gas in the panels, rather than the more common use of water (or antifreeze). The system works in the same way as the current system for fridges, although it does this in reverse. A refrigerant fluid is transformed to a gaseous state in the solar panel as it encounters the outside climate/temperature change (the fluid is heated), this gas is then transferred to a heat exchanger that increases the pressure of the gas, thereby creating even more heat (based on the laws of thermodynamics). This heat is then transferred to the water circuit. As the pressure is increased, the gas turns back into a liquid and is sent back to the solar panel to restart the continuous process.

The components of the Energie-Solar Thermodynamic system:

  1. Solar Panels
  2. Heat Exchanger
  3. Liquid distributing piping

The refrigerant fluid used inside the system is called R407c. It is a very stable fluid that has strong thermodynamic properties. It is also environmentally friendly should it ever escape into the atmosphere and is not a threat to people.

The Heat Exchanger is similar to those found in Ground Source and Air Source Heat Pumps, and the thermodynamics of the system work on a similar basis. To read more about these systems then please click here.

The solar panels are arranged just like other panels, with more panels in the system the greater the requirement for heat. Thus a small 4 block system will produce enough heat for a households water heating requirement, whilst a large 16 or 24 block system will produce enough heat for a large houses water heating and space heating requirements all year round.

The Basic System:



  • Solar Panel dimensions: 1 panel measures 2000mm x 800mm x 20mm, with a weight of around 8kg.
  • Heat Exchanger dimensions: 630mm x 860mm x 430mm
  • Panel location: The panels can be placed anywhere as long as they are not facing north. The angle tilt should be between 45 and 90 degrees for optimal performance, although this is not a strict rule.
  • Heat Exchanger: must be placed in a dry ventilated place and can be placed on the floor or hanging from a wall.

The systems currently available are:

Hot Water Cylinder Dimensions Input Output
Model Capacity (L) No Panels Height (mm) Diameter (mm) Power (W) Power (W) CoP
ECO 500 500 2 1830 650 595 2,800 4.7
ECO 750 750 4 2135 750 960 7,290 7.6
ECO 1000 1000 4 2185 850 960 7,290 7.6
ECO 1500 1500 6 2460 950 1,230 9,680 7.9
ECO 2000 2000 8 2520 1100 1,440 11,240 7.8
ECO 3000 3000 12 2900 1250 2,010 16,580 8.2
ECO 3000 E 3000 16 2900 1250 3,210 24,210 7.5
ECO 4000 4000 24 2960 1450 4,140 31,430 7.6
ECO 5000 5000 32 3030 1600 5,690 42,600 7.5
ECO 6000 6000 40 2 x 2900 2 x 1250 7,630 52,790 6.9

What does the above mean?

Let us assume we are looking at the ECO 4000 system of 24 panels. This has a power input requirement, i.e. the kWh of electricity that is needed to power the system to generate the Output heat, of 4,140kW and an Output power of 31,430kWh of heat. Thus, the coefficient of performance (CoP) is 7.6. That is, for every unit of electricity you put into the system, you get 7.6 times that amount of energy out of the system in the form of heat. Obviously this is a maximum CoP, and in practice the average working CoP is around 3.5. This is significantly higher than traditional Solar Thermal systems that tend to have a CoP of around 2 to 1. The coefficient of performance is affected by how much energy you need the system to produce. If you require the system to heat the water in the house to around 60 degrees (i.e. in winter), then the CoP is likely to be around 3.5, whilst heating the water to just 35 degrees (i.e. in Summer) will create a CoP of around 7. That said, more traditional methods of heating, such as immersion heaters, have a CoP of 1 or even less than this (meaning more energy input is required than the outputted heat). For example, Gas and Oil boilers have efficiency ratios of 85%, equivalent to a CoP of 0.85. This is also why the Energie-Solar system is more suited to under floor heating, a system that requires a lower temperature/more continuous flow of water through it, which increases the CoP of the system.

Other advantages of the system include:

  • Generates heat even with outside temperatures down to -10 degrees and therefore will work 95% of the time in the South of the UK (night and day), unlike other solar thermal systems that generally work only 60% of the time (only during the day).
  • Is efficient enough to generate large amounts of heat that will not only provide a households hot water requirements, but also provide their heat space requirements also.
  • Doesn’t have the problem of freezing pipes

Note that because this and other solar thermal systems generally require renewable friendly water tanks, it may be a requirement to replace your existing hot water tank with a Thermal Store water tank.

The one caveat of this, and other more traditional Solar Thermal systems, is that it will only heat your hot water to 55 degrees, whereas traditional gas boilers will heat the water to 75/80 degrees. It is for this reason that the system works best with under floor heating systems or low surface temperature radiator systems, rather than more modern radiator systems. That said, the system can work in conjunction with your old boiler system, which can be used to top up the water temperature, or provide the heat when the thermodynamic system isn’t producing heat. In fact there are many different ways the system can be adapted to suit your purpose, and Cernunnos Homes can help you achieve this.

An example of a Energie Solar Thermal System for Hot Water and Space Heating:


The Solar Thermal System and the Renewable Heat Incentive:

The details of the RHI have not been finalised, although some provisional details have been announced. We have applied the provisional details in order to see how the RHI will affect the cash flow of the Energie Solar Thermal System.

For a detailed overview of the UK’s Renewable Heat Incentive please click here

Some pictures of the system installed in a recent installation conducted by Cernunnos Homes:

The Heat Pump:


The Thermal Store:


The Roof Panels:

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The EnergieSolar Thermal Panels alongside some traditional Solar PV panels:


To view a Case Study for this project please click here.

To view the systems we offer please visit our online shop.