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Renewable Energy: Running heat pumps on off-peak electric

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    I like the idea of heat pumps because they allow you to "claw back" the inefficencies of centralised electricity generation and use electricity for heating. This has the benefit of centralising emmissions where they can be eliminated (by using emission free forms of generation) or captured using CCS. This is a key advantage over directly buring fossil fuels in the home in my view.

    However, running a heat pump 24/7 controlled by a room thermostat will increase peak load electricity consumption on the grid. This is something to be avoided in the UK where increases in peak load demand are supplied by fossil fuels.

    What I want to know is, can you run an air to water heat pump at periods of low demand (i.e. at night) and store enough hot water to heat a home during the day through rads or underfloor heating and supply the hot water demands? Will the COP be badly affected by using the cooler night air as a heat source? I guess a GSHP would work better but I'm put off by the installation cost of these things. I'm thinking in the context of back up heating in a well insulated property by the way.
    • CommentAuthortony
    • CommentTimeDec 13th 2007
     
    If it is well insulated how come that you need heating? :bigsmile:

    All you would need would be either a large enough thermal store, or a thermally massive building or high thermal mass associated with the underfloor heating.
    • CommentAuthorfuncrusher
    • CommentTimeDec 13th 2007
     
    chris; I've noticed we often have similar viewpoints so no surprise we have both thought about off-peak heat pumps for the same reasons. I personally am sceptical about air-sourced, because as you say the source temp is at its lowest at the very time demand is greatest. I think thermodynamically water-to-air should be most efficient, because groundwater is relatively warm when demand is at its peak, and low output temperatures of say 23 C would be fine. This is in principal much more efficient than a water rad system where required temperatures are over 55C for any remotely acceptable performance, but does require fan power. UFH at say 35 prima face lokks better than rads, but any gain is worthless unless the building is in continuous occupation.

    There are obvious drawbacks to forced air heating, but probably with careful design it could be done. However, any thermal store would have to be water based, so pumps and heat exchangers are needed.

    the popularity of air/air systems is because (1) they are cheap (2) they occupy no land (3) they can be integrated with aircon. Hence popularity in much of the US.

    I did quite a lot of sums on this problem. Salesmen always have their own agenda and will always specify equipment to cope with worst case scenario. This is rarely the most economic. You need to analyse daily max/min temp data (the statistical distribution, NOT the average) to determine the statistical distribution of the heating load, and then consider whether peak-lopping by other means is attractive. EG using off-peak immersion heaters to boost thermal stores. It might be possible to design heating systems with both UFH and rad loops, so that in cold snaps or when quick response is needed hot water goes thro the rads and having cooled then goes through the UFH.

    Something of the order 3cu.m of water will probably be suffice.
  4.  
    If we were just talking about back-up space heating in a super insulated house, then a cheap air to air heat pump in the main room might be all you need, particularly if the layout of the living areas is fairly open plan. In most areas there is an Economy 10 tariff which allows 3 hours at cheap rate in the afternoon (when it is warmest and the heat pump will work most efficiently), ideal timing to give the house a nice warm up in winter before the occupants get home from work.

    However, that is no help with the hot water (for which solar is no help in winter). If there was a similarly cheap, air to water heat pump you could run in the afternoon on cheap rate Economy 10, to heat up a tank of hot water, then you have a simple and cost effective method of supplying back-up heating and hot water. Anyone know of such a air to water device and its cost? Didn't they fit air to water heat pumps located in a solar sun space at Hockerton? I like the idea used at BedZed of an insulated airing cupboard, with no insulation around the tank - great for drying clothes and, if cleverly located upstairs, vents can be opened into adjoining bedrooms to deliver heat if necessary.

    As Tony points out, if the house is well insulated why do we need dedicated central heating systems? So I'm not really into UFH or radiators if they can be avoided. Just extra complexity and unnecessary cost on a project in my opinion.

    I'm all for simple, cheap, efficient solutions to heating and hot water because that should not be where the emphasis or the expenditure should lie. We should be focused on reducing the need for heat not installing expensive, "green", ways of providing it. The cost savings on the heating should fund the extra insulation and draft reduction work.

    Something like the above set-up would be a alternative to the combination of solar and a wood stove which might not work well in cities because of smoke pollution or lack of a suitable location for a solar panel.
    • CommentAuthordavid
    • CommentTimeDec 14th 2007
     
    Trianco, well known manufacturer of fossil fuel boilers make a range of air to water heat pumps at reasonable prices. Lots of info on their site.
    This the address of a company which sells them and shows prices.
    Sorry it's not a link.

    http://www.heatandplumb.com/acatalog/Activair_Air_Source_Heat_Pumps_5kW.html
    • CommentAuthordavid
    • CommentTimeDec 14th 2007
     
    Oh yes, it is a link!
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    Thanks for that David. They seem like the tool for the job.
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    If that really gave 5kW running at night, then you could store 35kWh in the Economy 7 period.

    According to the top hit on Google, the average person in the uk uses 14.1kWh hot water per day, giving you enough hot water for 2.5 people. Suitable for a small house, I'd say.

    Interestingly, it's only worth boosting it in the daytime if you can get a COP of over 10.04/3.94 = 2.54822335 as that is the difference in electricity rates. Might be simpler to just use an immersion heater at the end of the economy 7 period if you need to.

    It would be cheaper than gas (running costs for EDF energy southern region on Economy 7 - very naive estimate) if it had a COP of over 1.69(=3.94p/2.330p), which seems not unrealistic for the whole year. That said, I know little about heat pumps.

    I think that for hot water for small homes, this may be a goer. Of course, you have to ask how you're heating the place. Woodburner maybe?

    It may even be worth buying two ASHPs, as 35kWh is enough for 24h worst case heating of a passivhaus of 145m^2.
    I doubt you get 5kW when it's really cold, though, unless it switches to resistance heating.

    You can do a similar calculation for 9kW ground source pumps, and these look very favourable for decent sized houses, but of course the set up costs are huge unless you have a large lake or running water onsite.
    • CommentAuthorworkaholic
    • CommentTimeDec 14th 2007
     
    Take care with the Trianco heat pump. It is basically a swimming pool heater and I can only find references to it operating at 5 degrees. The HeatKing ASHP site mentions operation at -20 degrees for their heat pumps!
    • CommentAuthorPeter A
    • CommentTimeDec 15th 2007
     
    Hi Chris,
    For quite some time I have been a fan of air to air heat pumps, I quite like the NIBE products that also use exhaust air to aid the performance, but I have always been a bit concerned about their ability to generate hot water, heating's not a problem if the home is well insulated and has a good HLP. I think they need to work in tandem with another renewable or possibly even two, wood for the winter and solar for the summer? I have been looking at phase change materials that will store the surplus energy and then assist hot water and heating when sources are struggling, came across www.pcmproducts.net and a product called BallIce, perhaps this would work well with off peak power to store energy that can be called upon in peak periods?
  11.  
    I didn't realise the NIBE Fighter heat pump doesn't warm the incoming air (the heat in the exhaust air being used to heat the water instead), it is just drawn in through vents in the windows or walls. Might this be a good combination with supply air windows? That way the incoming air would be pre-warmed.

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