Thermodynamic Panels NAREC Tests

[The Solar King]

There are so many threads on this, thought I would start a new one.

First results of NAREC tests now posted here:

Thermodynamic panel test - Narec Distributed Energy

Early days, but interesting.

 

[Worcester]

mmmm..... Let's hope we can get the results over a long period

 

[solarfred]

Do we know the actual water usage rate ? Or how does it work in this test to reflect daily DHW consumption?

 

[Gavin A]

Do we know the actual water usage rate ? Or how does it work in this test to reflect daily DHW consumption?

I was wondering that - to be honest I'm less than impressed with this presentation given that Narec have had shedloads of public funding, not to even include a basic method statement, or details of hot water consumption levels etc is pretty bad.

From what I can tell, I'd suspect that there is very little hot water use from the cylinder, and they've got the system set to attempt to heat the cylinder to 55 degrees, so right at the top of the possible temperature range, and right at the least efficient point.

I suspect that it's mainly just keeping the temperature topped up between 50-55 degrees, compensating for thermal losses through the insulation - something they seem to acknowledge in this bit.

The COP rises to 1.68 on Jan 22nd due to an increased hot water demand (approx double) when conducting verification of the tap cycle control system; suggesting unit performance improves when subjected to higher loading.

As with all heat pumps, thermodynamic panels will operate at their least efficient point when heating to 50-55 degrees, so setting up an experiment in such a way that they're going to be operating at that least efficient point the majority of the time isn't exactly being fair to the technology.

I've backwards calculated from the 2.289kWh thermal losses, and 0.41 COP gain when including those losses, that the actual hot water use would have been in the region of 58l per day (assuming raising the water temp from 5 to 55 degrees).

The average hot water use per household given by the EST is 122l per day, so I'd at least have expected this experiment to have emulated the average conditions, rather than using half the average hot water consumption.

My suspicion would be that the system isn't really capable of achieving 55 degrees water temperature, as the top end of the standard heat pump range at the condensor is usually 55 degrees, and there has to be some level of temperature differential between the condensor and the water for the heat to be transferred. This would account for the really bad COP levels, as the unit will essentially be running for a fair amount of time to achieve the last few degrees of temperature rise.

If we were setting this up, I'd be setting the target water temperature to a maximum of 50 degrees to give the thermodynamic system a fighting chance of achieving some reasonable efficiency figures while still avoiding any issues with legionella, and ensuring at least 120l a day of hot water consumption at average consumption patterns.

 

[TomBradley]

Thanks for your interest in the thermodynamic panel test. Rather than answer questions on forums etc. we have begun a FAQ page on the test.

This can be found at: FAQ - Narec Distributed Energy Thermodynamic Panel Test

Email [email protected] with further questions and we will (eventually) answer them.

 

[Gavin A]

I've backwards calculated from the 2.289kWh thermal losses, and 0.41 COP gain when including those losses, that the actual hot water use would have been in the region of 58l per day (assuming raising the water temp from 5 to 55 degrees).

Ok I'll hold my hands up there, I seem to have accidentally calculated the hot water volume for the 2.289kWh thermal losses figure rather than the actual water heating figure, so this statement was wrong, and these figures now released by NAREC do look right.

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.

 

[TomBradley]

Ok I'll hold my hands up there, I seem to have accidentally calculated the hot water volume for the 2.289kWh thermal losses figure rather than the actual water heating figure, so this statement was wrong, and these figures now released by NAREC do look right.

Thanks. We'll keep updating the FAQ as the year goes on. Hopefully with a more detailed explanation of tapping cycles it will be clearer to our website visitors what the test results mean.

 

[Gavin A]

Thanks. We'll keep updating the FAQ as the year goes on. Hopefully with a more detailed explanation of tapping cycles it will be clearer to our website visitors what the test results mean.

cheers.

While you're still here, I think the most useful things we could extract from this test would be to know at what stages the COP is higher and why, which really needs the data on the graph to show the water temperature at the time, alongside the electrical input if possible to know whether the unit is actually working or not.

The overall daily figures are useful, but really the best use of this test from where I'm standing would be to actually inform best practice to achieve the highest COPs rather than just determining what COP the manufacturers recommendations will achieve.

ie the 2 most interesting points on the graph are the 2 peaks in the COP levels, but it's impossible to know what they mean without knowing the temperature the system was running at at these points.

any chance of that water temperature data being added to the graphs?

ps thanks for the FAQs, sorry if I prodded a little hard, it just niggled me not to have a method statement included.

 

[Gavin A]

ps as a fyi, we have 2 systems we're about to install as test rigs ourselves, so in a couple of months time you can pick holes in our methodology as well

 

[The Solar King]

Final test results have now been published.

Thermodynamic panel test - Narec Distributed Energy

I will leave you to draw your own conclusions. Doubt they meet the EU definition of renewable heat due to low average COP. If these results are typical, I cannot see how the equipment could ever pay for itself.

 

[Gavin A]

I was at our test site the other day, unfortunately I couldn't manage to get real time monitoring of both the heat and electricity consumption, so can only provide an average COP for the period.

So for the last 3 months or so, the average COP was 2.25, with 1741.9kWh heat generated from 773kWh electricity consumed.

This system is supplying heat to a swimming pool, so operates at a lower temperature than hot water systems would.

It is however in a position where the 2 panels are probably fully shaded at this time of year, and for much of this period. I'll have to check the situation next summer. One of the panels is much more shaded than the other as well (the customer insisted on having the extra panel).

Looking at the monitoring data though it is noticeable that the heat output is very dependant upon the outside air temperature, hitting 2kW at a point where the outside air temperature is at 20 degrees, but dropping to 1.4kW when the air temperature drops after dark.

So I suspect the summer COP will be a lot better.

One problem though is I think the system hasn't got the capacity to extract more than 2kW heat, and doesn't have the capability to reduce compressor power, so I think it ends up cycling to prevent it over heating when there's actually more energy available than it can cope with, which would explain the low COP figures from Narec even in peak conditions. Not enough high temperature data in this data set though to be sure of that.

If these were inverter driven, then I think the COP's in summer would be a lot better. There has to be a reason why the results don't match up with the theoretical possibilities.

 

[Worcester]

End of the day though, wouldn't an air source heat pump just have been a better solution - especially for a swimming pool with it's low flow temperature?

 

[Gavin A]

End of the day though, wouldn't an air source heat pump just have been a better solution - especially for a swimming pool with it's low flow temperature?

they'd already fitted a gas boiler for the pool, this was supposed to be supplementary to make efficient use of excess solar pv.

 

[TomBradley]

cheers.

While you're still here...

Apologies that we never replied, not here much. We'll put up a summary of the thermodynamic test when we have time, work has been overwhelmingly busy for the past year.

 

[Gavin A]

no worries Tom, ditto.