Solar Voltaic Panels & Air Heat Pump Combination
Personal experience in Glasgow, Scotland, of combining a 4kW solar voltaic panel system with a small air-sourced heat pump, a small low-emissions wood stove, and an existing gas boiler, to achieve radical savings in carbon footprint and energy costs. Information previously given on this page is now published as an article in the journal, Reforesting Scotland, a PDF of which is provided in item 1 below.
Update to April 2015: Just to say that I'll update all my spreadsheets as soon as I get the chance (busy with book deadlines), but basically, my figures for the the year 2014-2015 have come out if anything slightly better than they were for the previous year. Hereafter it will be less easy to give accurate comparative figures, because we've now added external wall insulation helped by a Scottish Government grant scheme. To summarise: our domestic carbon dioxide footprint has fallen from 5.4 to 2.2 tons per annum, and our combined electric/gas bills have fallen from £1,400 to just over £100 per annum. Without the FIT subsidy of about £600 per annum, our bills would have halved to £700. However, that's misleading, because we export half the power that comes off our roof and get paid for it only a third of the price at which we purchase. If this was adjusted for by shifting to a Net Usage Basis (where we'd pay only for the power we use net of exports), then in rough figures if there was no solar subsidy our bills would still be showing a reduction of about three quarters. Plus, the heat pump we're using in our domestic configuration works so well that the house is a lot warmer than it was previously. I'll update all this as soon as I get a chance, and our address is 26 Luss Road, G51 3YD, in case anybody passing by wants to pop their head over the hedge where the heat pump resides and see our system for themselves.
1. Download the Reforesting Scotland article: Wood, Wind and Sunny Govan (Oct 2013, 5 MB) comprising 2 pages of the story, plus 10 pages of detailed technical endnotes including energy, carbon and cash return calculations (with DCF). This article is the central reference point for what I'm describing here. Note that Reforesting Scotland is not responsible for the accuracy of the article or the claims that I make in it.
2. Monthly data logging Excel spreadsheet. This shows my calculations for gas and electricity consumption, my solar output, cost and feed-in tariffs (thus net energy costs) and carbon footprint calculations. My solar panels went in on 20 January 2013, and the air-source heat pump was added on 2nd April. As such, this table, showing both, runs from the start of April 2013. I have also kept daily data logging for the months of April 2013 and May 2013. Feel free to take and use these tables for your own logging, modifying input data as necessary, and let me know of any errors you might spot.
3. Monthly solar output data and FIT: This Excel spreadsheet dates from 20 January 2013 when the panels were installed and, month by month, charts my solar energy production against the Met Office sunshine figures for our nearest weather station - Paisley which is about 3 miles away. This lets me see how far underages or overages in my solar energy output been a function of variances from average sunshine hours. It also shows my solar FIT calculations.
3. Carbon & monetary costs 4 year historic data: This spreadsheet was what I created to calculate my historic energy cost and carbon footprint (domestic), based on enegy bills for the previous 4 years.
4. Carbon profiling model: This one's for energy nerds only! Because I paid for my gas and electricity on fixed sum monthly basis, with bills only twice a year (on a green tariff until Scottish Hydro and the RSPB terminated their partnership), I lacked the data to judge my monthly energy consumption. On installing the panels and heat pump I wanted to make before and after calculations. This spreadsheet uses average monthly temperatures from Met Office stats to back-calculate a proxy figure for a baseline monthly energy use and therefore, carbon footprint. I suspect that it overstates prior summer consumption and understates winter consumption, and therefore my very high carbon savings on baseline over the period covered in the Reforesting Scotland article will very likely be cut come the winter. It was the best methodology I could think of at the time, and inasmuch as it may be a bit out it won't matter over the fullness of the year.
I've just updated my solar and heat pump figures to end of December 2013 (installed 20th Jan). In round figures, we produced 3,500 units off the roof in 2013. Our previous average electricity use was 4,000. We've therefore had 88% of the previous baseline provided by nature. We over-produced on projected figures by 12.9%, but Met Office figures for Glasgow show the year was 14.6% sunnier than average.
The heat pump was installed at the start of April. From then to end of December 2013 our net combined gas/electric cost as calculated directly from meter readings, with daily standing charges added, was, in round figures -£70 (yes, minus, because of the solar Feed In Tariff (FIT) payments.
Let me try and adjust my figures onto an annualised basis assuming that the coming months of Jan - Mar will average the same as Dec has just done. December's energy costs before deduction of FIT were £104, so Jan-Mar should be £320 in round numbers. (Not bad - winter energy at less than a packet of 10 fags a day). Knock off the £70 credit and that's annual cominbed energy costs of £250 - £21 a month whereas previously we were paying £117/month (£1400 a year). We're therefore saving £1150/annum - our costs have been reduced by 82%. Remembering that 2013 was 15% sunnier than average, let's say we're saving £1,000 a year, then the payback period on our £7,000 investment for the solar panels and the heat pump equates to 7 years.
What about the argument that these green technologies rely on FIT, which is arguably socially unfair because it the solar aspect relies on have a south roof so not everybody has equal access. Good point. That's why in my Reforesting Scotland article I proposed the NUB - net usage basis - that instead of phaffing around with FIT payments, panel prices are now so low that householders should be compensated only for the net energy they export to the grid, but on the same price as they pay on importing (13p/unit in our case). So, let's calculate what would have happened in 2013 (with Dec - Jan adjusted in as above) if there was no FIT but only the NUB.
Onto my £250 annual costs, pay back the £620 FIT for 2013 and our gas/electric costs would be £870. Adjust for 50% of solar power produced being exported (most is in summer, when our consumption is very low, so that's a fair estimate), and we can reduce that figure by our annual production, 3,500 units, halved and multiplied by 13p = £230. Deduct that from the £870, and our energy costs for a year would then be £640. That's a 53% fall on previous costs without any subsidy from anyone. The carbon footprint reduction calculation on the same assumptions, calculated by spreadsheet, is 58%.
Given that 2013 was a sunnier than average year and so there's a need to downplay those figures, I can reasonably say that the combination of solar panels with the air-sourced heat pump has reduced our carbon footprint by more than half, our domestic energy costs by about three-quarters, and with a payback period on the £7,000 investment of 7 years. Even if there had been no solar feed-in subsidy our combined energy costs would still have fallen by about half, with a payback period on costs of about 10 years. I think that this is saying that domestic renewable energy has come of age.
Oh, and there's one other thing - I have a French wife who has always shivered in what she called "cold and rainy Scotland". Although it makes my carbon reduction figures less impressive than they could have been, the whole house is a lot warmer with this heat pump running steadily through each day. She's very happy!
(Author of Hell and High Water:
Last updated: 17 June 2015