My PV Solar/Heat Pump Performance

supastar72

My solar PV system has now been operating for over a year and readers might appreciate the performance figures. An air source heat pump was installed a few months later.

My system consists of fourteen 260w HJSOLAR panels giving a total installed capacity of 3.64 kWp and a net capacity, allowing for shading, dirty panels, wiring losses etc., of 3.14 kW. The panels face SSE and are inclined at 45 deg. Due to shading from the neighbour's roof, the panels were installed in two strings of six and eight panels. These were connected to a two string Samil SolarRiver inverter, SR3K3TLA1, rated at 3480 watts. The generation meter is a Landis and Gyr E110. The whole system has a 10 year guarantee and it started operating on 9 Jan 2013 (but I was only paid from the 17th as that was the date I filled out on the application forms!).

From local meteorological data, I could expect about 1025 units of electricity from each kW of installed panels. So this gave a net potential of 3.14 x 1025 = 3220 units per annum.

To monitor the system I purchased an OWL intuition package which gave me online data storage and analysis of all the data. It also has an app for my Android tablet.

From the start it was obvious that the best value would come from using as much of the power as possible, as it is generated, rather than exporting it to the grid. Following research, I installed an immerSUN from day 1. This device senses when power is about to be exported to the grid and re-routes it to the immersion heater in the hot water cylinder. When the immerSUN shows amber, it is waiting for power. When it flashes green, it is diverting power to the immersion heater. When it shows permanent green, the hot water is up to temperature. So devices, such as the washing machine and dishwasher were operated in the green modes. The immerSUN can operate a second immersion heater.

On May 13, the superb 210 litre Dimplex cylinder, ECS210HP-580, was operating. Apart from heating it initially with mains electricity using the manual boost mode on the immerSUN, the PV panels provided hot water right through until September 11. (That is four months of free hot water using over half a megawatt of free electricity.) This cylinder is heated, from cold mains water at 10C, by the immerSUN which raises the water temp to 72C (the max setting on the immersion heater), so nearly doubling the capacity of the cylinder. Losses from the cylinder are stated by the manufacturer as one unit per day, as it is so well insulated, but at the higher temperature and with real life lagged piping, the losses were just over two and a half units per day. When we went away, we switched the immerSUN off, so that the power was not wasted as heat loss but was usefully exported to the grid. This occurred for just over two months in the year. Although the cylinder is large for two people, it meant that sufficient hot water could be stored to give 4/5 days of hot water consumption in poor weather. In my opinion this approach totally avoids the need for a solar thermal system.

I also read that even a small film of scale can dramatically reduce the efficiency of heat exchangers and immersion heater elements. As we are in a hard water area and because the hot water would now be stored in a cylinder, I fitted a Timeworks AD11 water softener. I got this for £425, delivered, on eBay (£800 odd rrp) and it comes with all the valves and bits to fit it. It has a 2 year guarantee and a telephone data link which sends information to their HQ so faults can be diagnosed remotely. Clever!

Energy saved by the immerSUN was in four phases. Firstly, when the immerSUN was installed, my heating was by a gas condensing boiler with an assumed seasonal efficiency of 80%. So my gas units (kWh) from Ebico were at 5.06p gross or 6.33p net (no standing charge). Secondly, after the gas boiler was removed, my mains electricity was supplied by British Gas at a cost of 12.14p per unit (ignoring the standing charge). Finally, once the heat pump was installed, DHW was then heated by that. So, assuming a seasonal COP of 3, units saved by the immerSUN were then worth 4.05p with BG. This finally dropped to 3.89p with a new deal (see below). (Note: At a seasonal COP of 1.6, the heat pump was costing the same as the gas boiler.)

On July 1, I finally had the 9kW Samsung air source heat pump working. This was set up to provide a flow temperature of 45C which was adequate for both a bath and shower. The immerSUN then raised the temp above this. The heat pump feeds an underfloor heating system which had been installed in the house seventeen years earlier. There was also the hope that, in winter when it was cold but sunny, the PV panels would power the ASHP.

In the middle of November, we switched our electricity supply to EDF with a fixed tariff until the end of March 2015 at a unit rate of 11.68p (nearly 5% lower than the pre rise BG rate!) So the units saved by the immerSUN were now worth 3.89p. When we gave EDF our first meter reading in March, we were told that our annual cost for electricity would be around £524 against an estimated £900. They automatically adjust the direct debit payment and this came down from £74 to £34. So our PV panels are covering the total cost of our energy bills.

The system generated a total of 2,595 units in the year. The immerSUN transferred 843 units, just under a third of the total power generated. According to the OWL, 550 units were exported in the year. (The immerSUN does not transfer 100% and units would be exported when the DHW came up to temperature and when the immerSUN was switched off).

The results were not as good as expected and the winter performance was barely 60% of predicted. I believe this was because the shading was aggravated by the low inclination of the winter sun. The panels were never cleaned and there are water/dirt resisting finishes available. Manufacturers claim up to an increase of 20% in the output. But in the depths of the winter, in the 50 days from Dec 9 to Jan 28, the immerSUN heated the cylinder, however little, on 34 days, so at times, not only was the base domestic load covered but the heat pump also.

Although the FIT payments are index linked, because the system had been installed within 3 months of the tax year end, the figure of 15.44p for units generated, did not rise. But the export tariff at 4.50p did rise to 4.64p. (Such is state bureaucracy!)

Units saved is approximate as the Owl was switched off when we were away, so I've taken the ratio of the figures shown when the Owl was operating. This showed just over 21% of generated units being sent to the grid.

So the results were as follows;

Generation 2595 units at 15.44p £400.67
Export 215 units at 4.50p £9.67
1082 units at 4.64p £50.20
immerSUN 278 units at 6.33p £17.60
164 units at 12.14p £ 19.91
375 units at 4.05p £15.19
9 units at 3.89p £0.35
Saved 1219 units at 12.14p £ 147.99

TOTAL £661.58

The system cost £5,208 and the immerSUN, £350, totalling £5,558. (immerSUNs can be fitted retrospectively for up to £600.) So taking the complete system, the above total represents an annual net return of

661.58x100/5558 = 11.9%

or, 14.9% for a basic rate tax payer or 19.8% for a top rate tax payer. The payback period will be around 9 years.

reeac

Interesting post. We have a 3 year old 2.22kWp system which cost nearly £ 10K but earns the higher FITs for its consistent 2,200 kWh p.a. output. We have no Immersun but our bottom line is 11% p.a. return i.e. 9 year payback time ... same as the OPs.