Earth Notes: A Note On Solar DHW For Our Home

Thoughts of Hot Water from solar for our home 2008/2009...

[Note: as of 2010 we covered most of the remaining available roof space with solar PV, on the assumption that we'll be prefering 'instant' ASHP in any upgrade of our central-heating and domestic hot-water system, and overall the reduction in carbon footprint with thermal or PV is similar especially given that excess PV output can be pushed to the grid to displace others' energy demands.]

I've been through many iterations on how we might make use of solar thermal to reduce our carbon footprint, learning quite a lot on each round.

Possibly the biggest insight is that our property simply does not collect enough sunlight to cover DHW (Domestic Hot Water) and CH (Central/space Heating) in mid-winter, at least in part because our roof surfaces face east and west, not south. (Our whole ~40+m^2 roof will only receive an average ~20kWh/day mid-winter, when DHW and central-heating demand can average ~40kWh/day, though down from ~60kWh/day before conservation measures.)

In 2008 I estimated our DHW usage to be ~6--8kWh/day (~2.5+MWh/y). We actually consumed at least about 8--10kWh/day in gas most of the year, some of which was/is for cooking, and our non-condensing gas combi boiler (GCNo 47 393 06) is 78.6% efficient and with a nominal maximum heat output of 24kW. Our DHW boiler-exit temperature had been ~50°C for a while, eg 51°C at the bath tap full open on a warm August day but I dropped it to ~45°C at boiler-exit with no complaints and (say) a bath at 40°C is pleasant for me, 37°C for our toddler. The cold water mains input temperature of 19°C 2008/08/30 mid-morning indicates that a temperature lift of ~20K--25K is required in warm weather, and maybe 30K+ in cold weather, ie a ~50% increase in energy demand in winter. (See other mains sample temperatures.)

It might be possible to cover most of of our DHW requirements for most of the year from the 6m^2+ of space remaining unused on our west roof (incident insolation >2kWh/m^2/day from March to September but only 0.5kWh/m^2/day in December cf ~4.7kWh/m^2/day June and July, mean 2.6kWh/m^2/day, ie a total of 960kWh/m^2/year isolation, and with an assumed collection efficiency of ~50% maximum). The advantages of using the space on the west roof are that it is completely unshaded, and facing away from the road minimises planning issues.

After our second round of grid-tie PV in 2009, the final roof layout leaves 2m from the north edge of the roof to the edge of the PV support frame.

Given that we've now dedicated the east roof to PV (~18% efficiency), consider that in conjunction with a top-efficiency heat-pump (CoP ~5) then we could convert the collected insolation to hot water with similar or better efficiency (>~50% overall) using the grid and/or batteries as our short-term energy store and exporting summer excess energy to the grid rather than wasting it. We could also change to a time-of-day or 'smart' electricity tariff and boost water temperature when grid electricity is cheap/abundant, which is probably increasingly 'green' as the UK's grid gets 'greener'.

Our current (Potterton Performa 24) gas 'combi' is fairly new but because it is only in efficiency band D (more than 10% below the best condensing models) and is not able to accept solar preheated water (though that might not be true given a device such as the Grant CombiSOL for supplying DHW either direct from a solar-heated hot water tank (or thermal store) else diverted via an 'instant' combi with input from the hot tank mixed down to 25°C with a TMV as pre-heated water) then I am prepared to replace it. (This would also reduce our dependency on mains gas which is getting to be a more scarce and expensive commodity by the day!) Any heat-pump replacement would have to be a 'combi' too, ie providing hot water and space heating (using our existing radiators if possible). We don't really have ground collector space for a ground-source heat-pump (GSHP), so we'd want a high-CoP low-GHG-refrigerant air-source heat-pump (ASHP) such as an 'Eco Cute" CO2/R744-refrigerant model, which should be OK as London outside temperatures are rarely <0°C. The heat-pump should ideally be an 'instant'/'tankless' design to avoid "standby" (tank) losses from stored water heated with imported energy; holding solar pre-heated water in a tank with its standby losses is OK.) I wouldn't want the maximum usable heat output in winter much below (say) 20kW so that, for example, a bath doesn't take much longer to run than now, but I wouldn't want the maximum electric input to be much over 10kW (~43A) since our main supply panel is only fused for 60A for example; both might be acheivable simultaneously if CoP never falls below ~2.

Note that the highest efficiency (CoP) would be gained for an ASHP by running it during the day when outside air temperatures are highest. But the cheapest and most plentiful electricity is usually to be had at night when neither space- nor water- heating are needed. Note that ~1t of water in the core of the house (in an IBC?) could act as a heat store for one day's combined near-maximum mid-winter heat requirement for us (~30kWh) with maybe a δt of 25K (eg heat stored at 65°C+ cooling to 40°C) preserving a reasonable CoP even at the highest store temperature. A tariff allowing scavenging of 'excess'/'green' electricity whenever available, primed with a forecast of the next day's temperatures to choose how much heat to top up the tank with, and conversely using 'dynamic demand' features to switch off at moments of grid strain and demand peaks, plus a day's thermal store for all heating (but using only a small hot-water tank in summer), could work very well in money and CO2(e) terms, and be friendly to a grid with lots of intermittent generation such as wind. Also, local microgenerated electricity (such as from PV) can be used rather than spilled to the grid for minimum cost/CO2. A smaller store, a grid-friendly (and heat-demand) modulated heat output, and a more airtight property, could deliver a similar CO2/cost performance.

Basic System Specification

This initial system should support the following:

DHW demand is estimated to be ~6kWh/day as of the end of 2009 (down from about 8kWh/d a year or so before, from behavioural adjustments).

Available initial collector area is unshaded ~4m^2--6m^2 on west-facing 23° pitched (concrete tiled) roof.

Note that the house used to have a hot-water tank and still has parts of the old system in situ though not connected, such as the airing cupboard with hot-water pipes routed through it, roof venting, loft-tanks, and so on, which may make refitting a tank-based system relatively easy. The old tank cupboard raw capacity is ~500l though the old tank was maybe ~100l. The interior dimensions of the cupboard itself (there is also more space in a larger immediately-adjacent storge area) is 0.5m x 0.5m x ~2.15m.

The house is a 3-bedroom end-of-terrace wood-frame construction in London.

I would like to maximise mid-winter energy capture/efficiency when we otherwise consume most gas/energy, at the expense of collection efficiency at other times of year if need be, to try to do our best to minimise our winter carbon footprint, and to minimise summer overheating/stagnation. (This might suggest drainback and evacuated tube or plate collectors, for example.)

Heating with Radiators

Note that to drive the radiators (to replace the other combi function) generally the radiator flow water temperature should reach ~55°C, though when the outside temperature is below 0°C a flow temperature over 60°C seems necessary (as of 2009/12/19) to deliver heat to the living room fast enough to keep it above 15°C.

Revisiting Instant Solar DHW with PV + ASHP?

With high-efficiency PV and a heat-pump with a CoP around 3 suggested above then the overall capture of heat with a PV+AHSP system is similar to a simple solar-thermal system (though much more complex and expensive), though summer excess is shipped to the grid rather than wasted...

Could it match the instantenous performance of our current 'instant' gas combi? For example, as of 2010/01/21 in the evening I can run a bath or a shower at about 1l/7s (~9l/min or ~140ml/s) at ~38°C (from a cold mains inlet temperature ~8°C, ie a 'lift' of 30K/30°C). At about 600W/K (4.2J/K/ml * 140ml/s) that implies about 18kW effective heat output from our ~24kW, ~80%-efficient boiler.

Move to summer, and run a shower or bath at the same rate, but now with the cold mains inlet at a little under 20°C, ie a "lift" of only 20K, implying a heat requirement of 12kW.

With a heat-pump CoP of (say) 3 which should be easy to achieve in summer, that implies an electrical input of 4kW, which indeed is something that we hope to expand our PV beyond in 2010 (a max PV capacity of over 5kWp and inverter output over 4kWp). Given that we might generally hope for reduced demand in summer, plus better CoPs and long sunny evenings for our west roof for example, we might well be able to cover much 'instant' DHW demand much of the time.

Back in winter, to support our 18kW heat demand, even if the CoP was only 2 (and generally we'd want it over 2.5 to ensure overall carbon-footprint reduction compared to simply keeping the gas combi) then we'd need ~9kW electrical input to match current combi performance, which is certainly do-able (40A vs our 60A connection) though clearly not likely to be covered from PV at the time!

In the case that our PV system was backed by batteries of course, we might still reduce peak grid demand and keep it away from peak (CO2/kWh) intensity, or could achieve the same with a hot-water store instead.

April 2011 Thoughts

As of 2011 we seem to have DHW demand down to ~4kWh/d (ie about the equivalent of one hot bath) with maybe another 2kWh/d mean gas consumption for cooking.

If we aimed to get energy imports for DHW down to zero in high summer (without too much boiling/wastage other than holiday week) that implies about 1--2m^2 of collectors at between 50%--100% efficiency, or maybe 1m^2 on each of the east and west roof surfaces, which should still be possible with all our PV installed, with care.

Excess not needed for DHW in a 'combi' style tanked system can nominally help with space/central heating (CH) outside summer.

2m^2 should gather over 4kWh/day in June/July but likely under 0.5kWh/d in December when total heat demand (including CH) has been over 30kWh/d, and indeed there may simply be so little available insolation that a likely flat-panel collection system may deliver no support at all.

Although electricity generation with export of excess remains possibly best, some simple solar thermal on the side seems like a good idea; if mains failed our solar PV wouldn't work without an expensive battery system and in any case could not export excess for others to use, whereas a little battery power for circulation could possibly keep us supplied with warm water for washing. (With battery support for the PV we could possibly keep the heat-pump running for more DHW/CH, using the storage of electricty and heat together.)

September 2011 Thoughts

I have just run across the interesting Volther Hybrid PV/T c/o Newform Energy.

The PV/T idea has interested me for a while (eg Zen PVTwin), but this is starting to look practical, with the added twist of using a heat-pump with a decent CoP, though it may only be feasible for systems larger than the 4 panels (~8m^2) that we could accommodate, and may exhibit new exciting problems such as grinding to a halt for the few days each of the last few year that we have had snow covering our roof, plus tank losses that we are otherwise immune to currently.

It appears that a 4-panel PowerTherm system coupled to a 200l thermal store (with no heat pump, just a solar pump), and (say) the CombiSol to work with our existing combi, could capture a little over 800kWh of heat and ~500kWh of electricity in a year, corresponding to up to 0.37tCO2/y saved.

A ~2t thermal store inside or under the house might reduce the risk of mid-summer overheating (acting as a heat dump) and extend the DHW season by a month or so, since there is about that much 'excess' heat available in the middle two months.

The cost per tCO2/y saved looks similar to our PV.

That should take us firmly carbon-negative and remove all gas consumption for DHW (leaving only cooking) in the summer. This arrangement could allow us to eventually swap out the gas combi for an ASHP relatively painlessly. Given our current plumbing layout we could also end up with hot water at our taps much quicker than now, at least in summer.

January 2012 Thoughts

Justin Broadbent of Isoenergy visited (2012/01/23) he pointed out that a heat-pump would be unlikely to save money (but we ought to wait until June to see the RHI details), that an ASHP might get us an SPF of 3 if done right but that a GSHP with borehole(s) might cost the same and have an SPF of 4 (combi style with ~200l thermal store with direct solar inputs), and that it might cost us something like £18k all in.

(JB also thought that our ex-airing cupboard could probably accommodate a ~200l tank/store if I wanted to put one there, and rightly reminded me that we also ought to consider upgrading remaining rads to more-efficient newer ones and lagging all visible pipework if making this sort of expendure.)

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