Earth Notes: A Note On Solar DHW For Our Home

Getting it done...

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

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.

I estimate our DHW usage to be ~8kWh/day. We actually consume at least about 10kWh/day in gas year-round some of which is for cooking, and our gas combi boiler is <80% efficient (non-condensing).

It should be possible to cover all of of our DHW requirements for most of the year from the 4m^2+ of space remaining unused on our west roof (incident insolation >2kWh/m^2/day from March to September). The advantages of using the space on the west roof are that it is completely unshaded, and that facing away from the road should not need planning permission.

If we later add a second 4m^2+ collector on the east roof, and a (~10t) seasonal thermal store to carry heat from summer to winter, then we may be able to cover DHW year-round, though at great expense!

In any case, I want us to be able to fall back to our current 'instant' combi when solar DHW cannot supply our requirements. The current (Potterton Perform 24) boiler is fairly new and even though it is only in efficiency band D (more than 10% below the best condensing models) and is said not to be able to accept solar preheated water which would be most efficient in winter, I am not yet willing to replace it.

I propose to split the project into two parts:

• a relatively-standard installation to cover all DHW for 6 months per year, but with more expandability than usual, and
• a expanded collector area and thermal store for year-round DHW.

The first stage should give most value (in terms of energy/CO2 savings) per pound spent, and the second stage can be added when money allows and contains some more experimental features that should not be allowed to get in the way of installing and benefitting from the basic system.

Basic System Specification

This initial system should support the following:

• Heat-exchanger heating of mains-pressure hot-water (ie the tank water is static) to avoid Legionella issues, possibly with thermostatic value (TMV) regulation to no more than ~55°C for safety.
• Ability to fall back to our gas combi boiler for DHW as needed, and possibily able to mix to low enough to feed as pre-heated input to combi when fallback is needed.
• Ability to function in the absence of mains electricity supply (and in particular not adding any parasitic mains power load).
• Ability to supply all DHW during ~6 months per year, and make some contribution for the rest of the year.
• Large hot tank to cover low-sun days, eg up to ~500l, and to support more collector area later.
• Ability to be expanded easily later to double total collector area.
• Ability to be expanded easily later to use a seasonal thermal store.
• Ability to be expanded later for excess heat-dumping (and waste-heat recovery as electricity) and greywater heat-recovery, ie at least a couple of spare heat exchangers or ports in the tank top and bottom, and tank temperature up to ~90°C or higher.

DHW demand is estimated to be ~8kWh/day.

Available initial collector area is unshaded ~4m^2--6m^2 on west 23° 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, 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 evacuated tube or plate collectors, for example.)

Expanded System Specification

The expanded system would have added:

• An extra 4m^2+ of collector on the east roof.
• A seasonal thermal store (~10t) to cover winter use.
• A heat-dump mechanism to help avoid overheating mid-summer, possibly with facility to add a heat-recovery electricity generator.

Sources and Links

• The 40% House vision to deliver 60% carbon saving in the UK's residential energy use by 2050 (2005 report). See also the newer Home Truths report (Nov 2007) from the same group on how to deliver 80% cut in carbon emissions from UK homes by 2050, particularly with reference to their suggestion that homes will be net exporters of electricity and that there will be no base-load.
• Good summary of various SDHW types/mechanisms/characteristics such as closed loop heat exchanger, drainback etc, c/o Arizona Solar Center.
• The Micropower Council, a newly established coordinating association for various types of small scale generation.