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Heat pumps
Ground source or geothermal heat pumps are one of the most efficient heating technologies there are. Latent solar energy extracted from the ground is boosted to a usable temperature by an electric compressor. Because typically it takes one unit of electricity to deliver three to four units of heat, you get efficiency ratings of 3 to 400%, compared to 70% to 90% for a conventional boiler and 15% to 30% for an open fire.
Ground source heat pumps go hand in hand with under-floor heating. For instance, you run the system at night – taking full advantage of night-rate electricity – and the thermal mass of the concrete slab then slowly releases the heat during the day. As far as running costs go, that dependence on night-rate power is absolutely crucial. You cannot however rely on it exclusively. In a domestic setting, the greatest heat demand generally occurs towards evening, so a daytime boost is essential. Some vendors say these systems will need half daytime, half night-time running to work effectively. Others say a mix of two thirds night rate to one third day rate is sufficient. Ensure the vendor spells this out explicitly. “It’s under-floor in most cases because you can store it when you have it available.” says Peter Keavney of the Galway City Energy Agency. “A compromise can be made where you’ve got under-floor in the ground floor and radiators upstairs, but you would be looking at oversized radiators because of the lower temperature of the supply.”
What about longevity? How long can you expect these systems to last? “I would expect them to last about twenty years”, Keavney states. “They’ve a given lifespan of about fifteen to twenty-five years but I can quite comfortably say we’ll see a lot of them well into the future…The only moving parts that are in them is the compressor which can be replaced quite cheaply…I have one in myself since 1997, and it has never been opened to check it…Every year, I press the button and off it goes.”
As far as appropriate applications go, you need adequate external space if choosing a horizontal collector, so many suburban and urban settings are out. Moreover, once pipes have been lain and backfilled, you cannot subsequently use the space for building or growing anything with a deep root structure. Geothermal systems have become popular in office settings, where daytime occupancy allows greater dependence on cheaper night time running. In general, using geothermal in conjunction with under-floor, because you are not getting heat on demand, takes some planning. Conversely of course, once usage patterns are established, these systems facilitate a high degree of automation. Set the controls and forget about it.
The downside of a heat pump is its reliance on electricity. “Our electricity comes from fossil fuels so that’s where the technology falls down.” Says energy consultant Andy Wilson. “It doesn’t really give us any greater independence from fossil fuels and if it does, it’s extremely marginal. Although the heat pump might be 300% efficient...the power stations are only about 38% efficient.”
Evacuated tube solar collectors
If a heat pump technology is the preferred choice but the ground is unsuitable for a horizontal array, or if the space just isn’t available, a vertical collector is an option. Though the SEI grant available is higher than for the horizontal collector, this is a considerably more expensive option. Peter Keavney points out however that there may be circumstances where the economics stack up a little better in favour of a vertical array. “It’s very cost effective if you’re doing one-off houses in the countryside where they’re actually integrating the borehole with their supply of water, where you were drilling down anyhow and you’ve got a drill onsite.” The third option is the water source heat pump, in which the collector is installed and energy extracted from a body of water such as a pond or a well.
Other heat pumps
While horizontal loop systems may not be suitable in some tight sites, air to water and air to air heat pumps do offer this technology to smaller houses and apartments with space restrictions. The operating principles are essentially the same, except instead of extracting energy from the ground, it’s extracted from the air. In addition to extending heat pump technology to space-restricted situations, air to water pumps may also be suitable for retrofits, if only because you don’t have dig up an established garden. As before however, insulation levels in the house are absolutely crucial. Retrofitting any green technology in a poorly insulated, leaky house is a waste of energy and money.
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