Thermodynamic panels: the UK's first independent test gets underway
Posted by Tom Bradley on 4 June 2014 at 9:40 am
Recently there has been an increasing discussion of a new heating technology, known as solar thermodynamic panels (or solar-assisted heat pumps). They are sometimes marketed as a “solar panel that works at night” to quote one manufacturer. Essentially they are a type of air source heat pump, which uses a flat plate evaporator to extract energy from the air and sun, whereas a conventional air source heat pump uses a fan drawing air over a matrix of finned tubes.
At Narec Distributed Energy, we became interested in the claims by different manufacturers, some of which seemed especially ambitious, particularly since none appeared to be based on UK test data for a thermodynamic panel system. As an organisation that carries out a wide range of tests of renewable and low carbon technologies, we thought it would be interesting to test a system and post the results publicly.
In April last year, we issued a challenge through our blog: a manufacturer would provide and install a system and we would test the system for free and post the results online monthly. A UK manufacturer took up the challenge and installed a system here in Blyth in December 2013. The specific system installed is purely for the heating of domestic hot water.
The purpose of the test is to quantify the Coefficient of Performance (COP) of a thermodynamic panel system under UK weather conditions, specifically in this case north east England weather conditions. The Coefficient of Performance is essentially the ratio of the energy used by the system (pumps, compressor, etc) against the energy delivered into the hot water load.
The system is under test in our Thermal Test Lab at our office in Blyth. This lab has been used for various heating technology tests, including boiler efficiency, energy saving flow restrictors, and the impacts of integrating traditional solar thermal panels with combi boilers.
In order to ensure that the test is repeatable, and representative of a domestic consumer, we are using what is known as a tapping cycle to control how much water is drawn off, and when. The standards BS EN 13203-2 and BS EN16147 contain a number of different tapping cycles for the boiler and heat pump industries. We used Tapping Cycle no.2 or “M” for this test, which corresponds well with the domestic hot water load of a four-person household (estimated by the Energy Saving Trust at around 120 litres per day).
The tapping cycle ensures the hot water load is 5.845kWh per day; the actual volume of water drawn depends on the inlet and outlet temperatures – if the water is heated from 10 to 50°C, this equates to 124 litres per day. This means the thermodynamic system will provide the same amount of energy every day.
In order to understand the efficiency of the water tank and immersion within the thermodynamic system, the test was run for four days without the panel input. This allowed us to measure the efficiency of the water tank alone, so that we could separate the impacts of the panel.
Since January 2014 we have posted the results on our website.
So, what have we found out? Ultimately, it is still too early to pass judgement on the performance of the system, as we do not have a full winter/summer cycle recorded yet, but we have identified that the water tank used is particularly efficient.
The thermodynamic system average basic COP for January is 1.02 rising to 1.36 in April. Taking tank losses into account, the average COP in March is 1.87, with a maximum of 2.64. Daily hot water tank temperature averaged 52.6°C. We have not seen the COP of 7 claimed by some manufacturers of thermodynamic panels.
Even without the summer data, we can see that the thermodynamic panel returns approximately twice the energy of an immersion heater. Therefore it would be fair to say the technology may be suitable for people off the gas grid, although obviously there are other competing, more established low carbon technologies to consider such as solar thermal or biomass, instead.
With regard to properties on the gas grid, we still need more data to make a fair evaluation of how a thermodynamic panel compares with competing technologies.
So what are the next steps from this? When we have a full six-month cycle which includes summer and winter, we will be able to make a fair evaluation of the technology. There are improvements to be made, this is ultimately a young technology, and there is much that can be done to optimise it for the UK climate.
Update from Tom (9 June 2014):
"We have noticed there is some confusion over how the COP we have measured for the thermodynamic panel compares to that quoted for air source heat pumps. The thermodynamic panel system under test at Narec DE is for domestic hot water only, and is thus producing water at a temperature of around 50°C. Generally, air source heat pumps are used for space heating and produce water at a lower temperature (35°C to 45°C in the test standards). To put it simply, the higher the temperature produced by a heat pump (air source or thermodynamic), the lower the COP. Essentially, this means that comparing the COP of an air source heat pump used for space heating to the COP of a thermodynamic panel for DHW is not comparing like with like, and could lead to a consumer making the wrong choice."
From the blog
More information about renewable heating systems from YouGen
About the author: Tom Bradley works for Narec Distributed Energy, which is part of the UK National Renewable Energy Centre group of companies.
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