FAQ – Narec Distributed Energy Thermodynamic Panel Test

Why is there no information on how the test is performed?

This information is provided on the same page as the data (http://www.narecde.co.uk/thermodynamic-panel-data/). It includes the location, variables monitored and the tapping cycle used. There is enough information there for anyone to replicate the test if they have a good knowledge of testing heating systems in laboratory conditions.

How much water is used? / What temperature is the water drawn off at? / What draw off pattern is used? / What is a tapping cycle?

For this test, Tapping Cycle No.2 from EN 13203-2 is used. Tapping cycles are set daily profiles of hot water draws, including the start times and energy per draw, developed by industry to enable different appliances to be tested with exactly the same hot water load.

By using this methodology, the thermodynamic test is producing data that is comparable to other domestic heating system types. Narec Distributed Energy have used this methodology when testing solar thermal panels, heat pumps, integrated solar thermal/gas boiler systems and energy saving devices for gas boilers. Tapping Cycle No.2 means 5.845 kWh of energy is contained within the water drawn 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 unit under test is set to 55°C as per the manufacturer instructions; actual tank temperature varies between 49.4 and 57°C as it cycles on and off on the controls. The cold water feed average for February is 10.7°C.

Results for February (1st – 20th) show the daily hot water volume to be an average of 114.6 litres, with a minimum of 111 and maximum of 118. For comparison, a 2008 study by EST [1] found the mean household consumption to be 122 litres/day, using data from about 120 houses. The Government’s Standard Assessment Procedure, SAP 2009 [2] uses 36+(25*N) litres per day, so for 3 people this is 111 litres and for 4 people 136 litres.   The tapping cycle is therefore very representative.

If a lower set point were used, e.g. 50°C, then more hot water volume would be drawn to make up the 5.845kWh daily demand.

For more details, please look to BS EN 13203-2:2006

Why don’t you use the EST figures for a domestic house total DHW use instead?

Because the EST figures are not a tapping cycle, just a total amount of water per day. Also, this would mean the test would not be easily comparable with gas boiler and heat pump tests.

Why don’t you test the system at a lower temperature?

This would not be how the system operates in a domestic setting, and be against the manufacturer’s instructions. This test is intended to show how the system would perform if installed and operated according to the manufacturer’s instructions.

Don’t Narec get paid lots of money to do this?

All funding for Narec is for the testing of offshore renewable energy systems. Narec Distributed Energy is a separate commercial spin off company which only works on onshore renewable energy training, testing and consultancy; therefore do not receive any government funding. The Thermodynamic Test is paid for purely out of company funds.

Why are you doing this?

We believe it is important that independent test data exists to show how a thermodynamic system performs in the UK. At present this data does not exist.

I have looked at the January data, is this representative of normal system performance?

The January data has a low solar contribution. Thermodynamic Solar Panels are marketed as systems that utilise solar gain to supplement the heat pump. Therefore, we would assume as we approach the summer the system would perform better. However, we do not know if that is the case. Please keep checking back to our website as the year continues, and we will find how the systems perform under a range of solar radiation levels. It is too early to make any kind of judgement on the yearly performance of a thermodynamic panel, either positive or negative.

Have you made any changes to the system during the test?

In March we fitted a time switch to ensure the system mainly operates during periods of solar gain, rather than on temperature control alone. This should ensure the system runs during daylight hours, and there should be adequate hot water supply for the tapping cycle which runs from 07:00 to 21:30. In a normal domestic situation this may run the risk of cool water in the morning if there is a large demand late in the evening e.g. a bath at 9:30, but the user could adjust the timer to reflect this.

Is the COP very low compared to Air Source Heat Pumps?

Yes, but there is a very good reason for that. The thermodynamic panel system under test at Narec DE is for domestic hot water and thus producing water at a temperature of around 50°C, whereas 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.

With regard to testing standards, air source heat pumps for space heating and/or hot water are tested under standard EN14511-3, whilst heat pumps for domestic hot water units are tested under EN16147. These are different standards for heating systems used for different purposes. Comparing the COP of the DHW solar thermodynamic panel under test with an air source heat pump tested under EN14511-3 is simply not comparing like with like.

If you wish to compare the thermodynamic panel with a heat pump, then the domestic hot water unit standard (EN16147) is a much fairer comparison. It would be interesting to compare these units to conventional solar thermal, and Narec DE will be running a public solar thermal test in the near future.

References:
  1. Energy Saving Trust, “Measurement of domestic hot water consumption in dwellings”, Department for Environment and Rural Affairs, 2008
  2. Building Research Establishment, “The government’s standard assessment proceedure for energy rating of dwellings (SAP 2009 version 9.90)”, Department of Energy and Climate Change, March 2010 (equation “a”, p.166.)