If someone told you that there's a new solar panel that could heat your hot water 24 hours a day, 365 days a year come rain, shine or snow, would you believe them? It sounds too good to be true, and that always makes me suspicious, but that's what the evangelists for thermodynamic panels say.

So what are thermodynamic panels?

Despite being panels, they are closer to an air source heat pump than they are to a solar thermal panel. They are basically a freezer in reverse. Refrigerant enters the panel and as it passes through it absorbs heat from the atmosphere and becomes a gas.

The gas then passes through a compressor which increases the temperature and finally through a heat exchange coil inside the hot water cylinder. This heats the water in the cylinder to 55 degrees. It is estimated that about a quarter of the energy absorbed by a panel comes from solar irradiation, the rest from air and rain.

How much hot water do they produce?

They claim to provide 100% of the hot water requirements for domestic and commercial premises, swimming pools, underfloor heating and can make a contribution to traditional central heating. They come with a built in immersion which uses electricity to boost the water to 60 degrees every so often to avoid any risk of legionella.

How do they compare with solar thermal?

Solar thermal panels heat water in the panels, where thermodynamic ones heat a refrigerant. This allows them to harvest heat from the atmosphere, where solar panels depend on heat from the sun. The refrigerant enters the panel at -22 degrees C, so even on cold winter days they can absorb relative warmth from the air.

Solar thermal panels perform best on roofs that face between south east and south west. Thermodynamic panels can be placed on a wall, or on the roof. It's best not to have them facing north or north west, but they don't have to face south.

Are thermodynamic systems more like air source heat pumps?

Air source heat pumps have a fan running continually, which requires more electricity than the compressor in a thermodynamic system. As a result the thermodynamic manufacturers claim a much higher coefficient of performance (COP) of 3.5 to 4 - that's three and a half to four units of heat generated for each unit of electricity put into the system.

An employee of the Northern Ireland distributor for Energie (dpaddym1), writing on the Green Building Forum, said that his system had a COP of just over 3 in the eight months from January to August last year. A UK distributor of Energie panels has measured coefficients of performance ranging from 2.1 when the outside temperature was -6 degrees and the panel had snow on it, to 5.2 in July. It has measured an average seasonal performance factor of 3.5 to 4. There are no independently verified performance figures for UK-based systems yet.

Are thermodynamic systems designed for use in the UK?

The two main manufacturers are based in Portugal and Spain. The systems they manufacture have not been designed for the UK market. However, I understand that there are companies aiming to bring UK-specific products to the market this year.

Have they been independently tested in the UK?

No.

How many panels would I need for a domestic installation?

A four person household would need one panel and a 250 litre cylinder. It would cost around £5,500 to install.

Is it worth it if I'm on mains gas?

It depends whether you are looking at it from a financial point of view. Heating with gas is cheaper than with electricity. However, if you are worried about energy security or carbon emissions, then it might make sense.

I saw an ad saying thermodynamic panels produce "free hot water", is that true?

No. There is a cost involved as electricity is used to run the compressor. I've seen estimates of costs around £8 to £10 per month for a domestic system.

Will thermodynamic systems be eligible for the renewable heat incentive (RHI)?

The Department of Energy and Climate Change says it is: "gathering information on performance and on the establishment of standards for these products. At present there is not an established methodology to test and demonstrate their performance and so they are not currently supported under the RHI. DECC is in discussions with the MCS who are working on developing standards. There are no immediate plans to bring in support under the RHI until demonstrable performance standards are established."

Prior to November 2012 thermodynamic panels were eligible for the renewable heat premium payments due to their Solar Keymark accreditation. However, on 5 November Gemserve, the body responsible for the MCS scheme (Microgeneration Certification Scheme) suspended the registration of solar thermodynamic products within the MCS database.

This was for two main reasons:

1. The products tested and certified through Solar Keymark used water and glycol (as do solar panels). However, the products being sold in the UK use a refrigerant F Gas for the heat transfer.

2. Heat pumps have particular requirements under MCS concerning the co-efficient of performance and energy costs of running them, and it's not clear yet whether thermodynamic panels meet the standards. In addition, it is a MCS requirement that estimated performance is calculated using SAP, but these hybrid type systems are not fully covered in the SAP methodology.

MCS is working with a number of manufacturers to develop the required standards.

Should I invest in thermodynamic panels now?

The MCS says: "To be completely clear, this decision does not mean that the product cannot be installed in the UK, simply that they cannot be registered within the MCS database." However,until they are registered in the MCS database they will not be eligible for the renewable heat incentive.

Views differ on the product:  An installer I spoke to has some very happy customers. He has been in the renewables business for 10 years, and rates the Energie solar thermodynamic system as "the best heat-producing product" that he's worked with.

A consultant is less enthusiastic: "The idea behind the technology is almost right, but it is designed for another climate." He cites as an example of one of the resulting pitfalls an installation where ice built up on the panel to such an extent that the panel mountings collapsed under the build up. "It can be dangerous. I'm not such a fan as I used to be."

He also asks why anyone would buy products that are outside the renewable heat incentive and the green deal. His advice for anyone who wants to go ahead is: "go and look at no less than three systems that the installer has done and talk to the owners."

Having heard quite a few outrageous claims made for these panels at trade shows last year, my advice would be for buyers to be wary of investing at this stage, until there has been some testing in the UK, and standards for performance are set by MCS.

More information about renewable heating options from YouGen

Heat pumps information page

Solar thermal information page

Biomass boilers information page

Renewable Heat Incentive information page

Heating and hot water information page

If you have a question about anything in the above blog, please ask it in the comments section below.