Solar Thermodynamic Panels

Solar Thermodynamic Panels: Independent Test Challenge

There has been a great deal of talk in the energy industry over the past year on solar thermodynamic panels. Some marketing material and testimonials suggest very promising results, however it seems there is has been no independent testing or verification of these systems in the UK.

26 April 2013

As an independent test centre, associated with the National Renewable Energy Centre, we are offering free testing for a solar thermodynamic manufacturer who will supply and install a system for free at our headquarters in Northumberland.
Solar collector technology
Marketing literature often compares thermodynamic panels with conventional solar thermal systems, but there are key differences in collector design.
Solar thermal collectors have evolved over the last 30 or so years into mainstream construction products with EN and BS standards. Collectors are usually glazed and combine selective coatings and high insulation levels to maximise the flow temperature and optimise solar energy collection efficiency. In the temperature band for heating water (e.g. 20-50 K above ambient) test results show efficiency is typically 55 to 65% for glazed collectors. Unglazed collectors are rarely, if ever, used for heating domestic hot water in the UK because the efficiency falls quite dramatically with increasing temperature difference; although they are very effective for low temperature applications such as heating swimming pools in summer.
The technology is well proven and typically saves between 50 and 70% of water heating energy; a 4m2 system will typically deliver around 1600 kWh of heat at the tank, using about 45kWh of electricity in the process. Back up heat is usually provided by gas boiler.
Thermodynamic panels are usually unglazed flat plates and use refrigeration technology to boost the flow temperature to the heat the water. It can be thought of as air source heat pump (without the fan) that can take additional gains by collecting solar energy if it is suitably orientated. Thermodynamic panels have the advantage they can produce heat from non-optimal collector orientations, or even at night, because of the temperature uplift the heat pump compression cycle offers. The key question is, how efficient is this process and what carbon saving benefit does over the existing water heating system bearing in mind the current UK grid electricity carbon content?
COP (coefficient of performance)
Thermodynamic systems use electricity for the heat pump compressor and circulation pump. Therefore, like any other type of heat pump, they will have a COP (coefficient of performance), which is a comparison of the energy taken to run, against the output heat energy from the system.
The COP of air-source heat pumps typically lies in the range of 2 to 4, depending on the weather conditions and season. Lower values are usually associated with domestic hot water production because of the high temperature uplift required; the higher values for spring/autumn space heating. Many heat pump suppliers recommend a hot water set point of 48 to 50°C to improve performance, with a regular pasteurisation cycle to safeguard against legionella (often using an immersion heater to 60°C once a week or fortnight).
EN standard heat pump testing requires COP testing at several temperature conditions and when interpreting these results it is important to compare the same conditions; e.g. A7W35 means water temperature of 35° and ambient of 7°C, for which one manufacturer quotes a COP of 4.13, falling to 2.80 at A-3/35 when the ambient is -3°C. This shows the COPs that can be achieved with a suitably designed low temperature heating system, such as underfloor in-slab. However, heating domestic hot water to 50°C requires a much higher temperature gradient, so it is reasonable to assume the COP will fall.
Some Thermodynamic suppliers suggests the COP lies in the range of 2 to 7 – the question is for UK water heating which end of the range is most realistic?
MCS
Thermodynamic panels have been temporarily suspended from the MCS installation database. This doesn’t mean the systems cannot be installed in the UK, but that they will not qualify for any financial incentives etc. which are for MCS approved technologies only.
The reasons given by Gemserv, who run the MCS scheme are:

• The performance of the products being installed in the UK cannot be fully determined; this is mainly due to the fact that these solar systems are being installed using refrigerant but the 12975 testing and certification through Solar Keymark did not use refrigerant as the heat transfer medium.
• Systems with a compression heat exchange unit are unable to meet the requirements for completing the SAP and performance estimation calculations required under MCS, for example hybrid type systems are not covered fully within the SAP methodology which is required to be completed under MCS.
• It is not clear how compliance with Part G of the building regulations is fully met, due to the system’s packaged control strategy. Installers are required to meet all parts of the building regulations under MCS, and it is uncertain if installers are able to do this with the system’s packaged control strategy.
• It needs to be determined if the classification of these systems within MCS are actually Solar Thermal systems or if they should be classified as Heat Pump systems.

Independent test offer
We therefore have a challenge for thermodynamic panel manufacturers. We are willing to carry out an independent test on a thermodynamic panel system, if provided with one fully installed with all required fittings at our test facility in Northumberland, free of charge by a manufacturer.
We are an organisation completely independent of manufacturers, and part of the National Renewable Energy Centre group of companies, so will give completely honest and realistic results. Whatever the results, positive or negative, we will release them on our website and to our press contacts. Let’s find out how it compares.