It's time to take heat pumps seriouslyBy Eric R Hawkins
Posted on 25 May, 2006
Article published in PHAM : February 2006 Issue (reproduced with permission of the publishers of PHAM News)
A range of air to water heat pumps is the latest addition to the range of alternative heating solutions from Powertech. Technical Director Eric Hawkins offers a guide to heat pump technology and its potential use in the UK.
A heat pump is essentially a refrigerator working in reverse. Simply put, it uses the air outside to provide space heating and hot water. In operation it draws air into one side of the heat pump and dumps out the other side the air at a reduced temperature. Heat pumps have been available for many years, but new compressor technology recently introduced into the marketplace has dramatically increased efficiency levels, thus making it a financially rewarding technology across many sectors.
Heat pumps take heat out of the air and convert it to hot water through a plate heat exchanger after being passed through the compressor, to be used as hot water for showers/swimming pools and space heating. They are particularly effective when combined with a solar thermal collector. With these two technologies combined, along with an energy efficient thermal storage system, the gas/oil boiler is redundant in new homes.
A remarkable thing about heat pumps is that they can extract heat from incredibly low temperatures. It would not be unusual for a heat pump to continue to work in temperatures as low as -5°C, except in these days of global warming, the loft temperatures and garages outside any wind chill rarely fall below 5°C even if freezing outside.
Where to fit one
An ideal location is in the loft space, hidden out of the way. There is usually a large amount of ‘wasted’ heat to be found there. The added benefit of transferring heat out of the loft and into a hot water tank is that it cools the loft space, reducing the ‘stuffiness’ of the home. Alternatively, the heat pump could go outside in the garage or in commercial buildings, plant rooms and commercial kitchens.
Efficiency levels
For every unit of electricity used to run the heat pump, the heat pump will give you between 3 to 4 units of heat back in return. The ratio of energy in to energy out is called the ‘co-efficient of performance’ (COP). The higher the COP the more effective the heat pump is at transferring heat to the place where you want it. To put this in context, a traditional gas boiler has a COP of around 0.65, compared with a heat pump of around 3. This makes the heat pump over four times as effective as a traditional gas boiler.
Ground source heat pumps
The air to water heat pumps that we supply are the real alternative to ground source heat pumps. A ground source heat pump system involves digging up large areas of a garden, drilling a deep home or trench to lay down hundreds of metres of pipe, all at great expense. It is said that the temperature 2-3 metres below the ground surface has a stable temperature of around 15°C, but if this were the case why is our cold water in the six months of winter arriving at the kitchen tap at around 6°C?
Our air to water heat pumps essentially work on the same principle, but without needing to do any of the above. Why take heat out of the ground when it's easier, cheaper and more effective to take it out of the air and at cost of 80% less?
Another problem with ground source heat pumps is how long will it be, after extracting heat from the ground before the temperature remains colder, given the limited summer season to replace the heat drawn out over a period of eight months.
There are key difference between the two types of air to water heat pumps – one is the type that has been designed to be fitted outside, for use in areas such as heating swimming pools in warm climates, the other type is for internal use, in attics/lofts of homes and buildings, garages, commercial warehouses and buildings with high roofs where heat gets trapped.
Linking up
In 2005, we started to combine the hot water generated from an air to water heat pump with the hot water generated by our Apricus vacuum tube heat pipe collector. Having already designed and fully developed the Powertech Multi Fuel Thermal Store to operate for both heating and hot water at mains pressure at 60°C, and established the Ply Pipe underfloor heating system in the market, it now seemed the right time to offer an alternative to the conventional gas/oil boiler central heating system.
What held back this hybrid alternative was the higher temperature (up to 80°C) which all thermal stores need in order to meet both the space heating load and that of hot water at mains pressure at the same time, during the winter months.
Like all combi boilers which also suffer from lower temperature and flow rates during six months of winter when the incoming cold mains drops to around 6°C, all thermal stores suffer much the same problem. Here at Powertech we identified the problem and made a few design changes. We sadly had to increase the cost to achieve the 60°C temperature our Multi Fuel Thermal Store operates at, but this allowed us to meet this temperature of 60°C through the two sizes of air to water heat pumps we now offer alongside our high efficiency Apricus collectors, with the back up of a 6 and 3 kW immersion heater.
Running costs
Electricity use is no more expensive than gas or heating oil when the servicing and maintenance of boilers/burners is taken into consideration, along with the eventual replacement costs. The 6kW immersion heating element supplied incaloy is wired up to off peak/night time rate electricity tariffs, giving the home owner seven hours of low cost energy to heat the 300-500 litre Multi fuel thermal store.
On the cheap tariff switching off, the air to water heat pump starts up and by this time of the morning the 6kW immersion heater would have heated the thermal store to 60°C. At 35°C it would have switched on the circulating pump to the underfloor heating or low water content radiators to meet the home’s early morning heat needs, as well as provide hot water in the top half of the thermal store for the morning’s washing and shower needs.
The smallest of the air to water heat pumps is rated at 4kW and uses 1kW to deliver 3kW of heated water up to 60°C depending on the air temperature being drawn in.
In order not to use more standard rate electricity between 7am and 10pm, costing some 0.7-8p a kWh (around £1.00 a day), we insist where possible that at least 11 Apricus vacuum tube heat pipes for every 100 litres of water storage be included in this hybrid system.
Solar collectors such as the Apricus brand have been proven time and time again across the world, from the freezing depths of Canada, Scotland and Sweden, to provide a substantial amount of 100% free, clean renewable energy direct into the thermal store, thus speeding up the temperature of the thermal store to 55-60°C, so the heat pump shuts down or maintains the desired temperature in the home through the day.
Ready boost
If a boost is needed after the sun disappears to the west, the second 3-6kW immersion heater can be set to come on through a programmer during the depths of winter when the solar energy gain is at its lowest.
We also recommend that in the lounge of the home a focal fireplace using electric or gas is installed as a boost for that room if it becomes very cold for a few nights during winter, rather than change any of the controls already in place.
Service costs
The ongoing servicing cost of a Powertech system is less than £10 a year, as once installed to the recommendation supplied, the only items over the 15-50 year working life of the system to maybe fail are the pumps and controllers, all of which are low cost and simple for any person to change, unlike that of a condensing gas boiler or oil fired boiler with burner.
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