Last autumn our annual bill for oil had reached £2000 so we began to investigate alternatives. LPG seemed to be even more expensive, and the lack of a gas supply left electricity as the only alternative, but as it is also expensive we investigated a combination of photovoltaic solar panels to generate electricity and heat pumps to use it to heat water.
Refrigerators and deep freezers are examples of heat pumps – they pump heat from the inside of the appliance to the grill or heat exchanger at the rear where the heat escapes into the air. The heat pumps that are alternatives to boilers pump heat from outside the building to the inside. There are several different types depending on the input and output of the heat pump. The heat can be taken from the air (air source) or the ground (ground source). The latter requires either a long deep trench in the ground in which is laid a pipe containing water or a deep shaft with a vertical rather than horizontal pipe (these are called geothermal). The heat pump takes heat from the water in the pipe, the water then being warmed again by being heated by the ground. These are expensive to install and require suitable ground which we do not have. An air source heat pump takes heat from the air which is sucked by fans through a heat exchanger. The output of the heat pump can be either hot water used to heat radiators and provide domestic hot water, or warm air piped through the house (the latter can also be used as air conditioners in hot weather). As we already have radiators we went for the hot water output.
Another difference between heat pumps is in the gas used. The heat pump works by compressing a gas which increases its temperature. The hot gas flows through a heat exchanger which heats either water or air which is pumped into the house. This cools the gas which is now allowed to expand cooling it even further (typically well below 0C). The cold gas flows through another heat exchanger which warms it using either the outside air or water in the case of ground source. There are two main types of gas used. Most heat pumps use refrigerants similar to those used in refrigerators and deep freezers. However these have a limited temperature range. They are suitable for ground source where the water never gets very cold and under floor heating systems where high water temperatures can damage the piping. The size of radiators needs to be doubled from that designed for a conventional boiler as the water temperate is lower. The outside air used in an air source heat pump can be very cold. So we went for R744 as the refrigerant. This is compressed carbon dioxide which poses no danger to the environment if accidently released, and will still extract heat from air at temperatures below -25C. It also produces very high temperatures – we get over 90C after compression with an outside temperature of 0C.
The efficiency of heat pumps is compared to that of an immersion heater which has an efficiency of nearly 100% in converting electricity into hot water. The typical value for a R744 heat pump is 300% (this is also called the Coefficient of Performance or COP=3) at 15C. This means that three times as much heat is produced using the same amount of electricity as an immersion heater, or the cost of heating water is one third of that using an immersion heater. The COP value varies according to the input and output temperatures. In our case it drops to 1 with an outside temperate of -30C and exceeds 4 at outside temperatures of over 25C (giving very cheap hot water in the summer). There is a sudden fall in the COP value from 2.6 at 5C to 2.2 at -1C because ice forms on the air heat exchanger and this must be melted by a defrosting operation (diverting warm refrigerant through the heat exchanger). However at temperatures below freezing the air is usually much dryer and so the fall with temperature is less. Ground source heat pumps have a more constant COP value typically between 4 and 5 because the input temperature is more constant, and they do not suffer from icing up - in practice they often rely on immersion heaters to boost the water temperature and the practical COP value is usually about 3. We have changed the wiring to our immersion heater to give us manual control. It will only be used to provide faster heating from cold in winter (i.e. if the heating has been switched off for a week or so).
The heat pump is most efficient when raising the water temperature in the output heat exchanger by 20C, and by limiting the maximum temperature of the water in the tank to about 65C. This is hot enough to require a thermostatic mixing valve to reduce the temperature to about 45-50C for washing etc. Ideally the hot water tank should be tall enough to sustain a temperature differential of 20C - this is about 3 feet or 1 metre between the pipes to the heat pump.
We have kept our existing radiators, but replaced the hot water tank with a much larger double tank. This has an outer tank containing the heated water from the heat pump, and an inner tank containing the hot water for domestic use. The radiators get their water from the outer tank. The reason for going for a large hot water tank is that the electricity generated by the solar panels during the day is used to power the heat pump, and the hot water is then available for use at night. This means we export a minimum of electricity to the grid (we are paid 3p per unit for exported electricity but have to pay 11p to import it again when we want it – converting it into hot water saves 8p per unit).
We have kept the same target indoor temperature of 20C as we had for the oil boiler. However most of the eleven radiators (one per room) now have thermostatic valves. The heat pump's power consumption varies from 2.86kW at 15C to 4.93kW at -5C which is its maximum. The latter corresponds just over 10kW output (COP = 2.1). Our oil boiler was double this and that could just hold the indoor temperature at 20C when it was below freezing outside. Our new installation struggles to keep the indoor temperature above 17C so extra electrical heating is required when it is below freezing. This could be provided by the immersion heater (the system is designed to do this automatically), but we have found an electric heater in the room more cost effective. This is not required when it is above freezing outside.
The heat pump looks like an air conditioning unit which either stands on the ground or is attached to an outside wall. It does make some noise, but not enough to be noticeable. The fan speed is much lower than air conditioners, and it can only be heard from a few feet away. The heat pump was installed at the start of January 2011 and completed on 11 January so the values before that in the spreadsheet are not reliable. As I have no accurate measurement of either the amount of oil used or the electricity consumed I have made the following assumptions. The price of oil used per day is assumed to be £5.47 constant throughout the year. This is equivalent to £2,000 per annum which is what I would have expected to pay in 2011 from our consumption in 2010 (this is based on £0.55 per litre). I have also assumed that the heat pump and the additional heater use all the power in excess of 15 units per day – measurements in October-December 2010 showed that our average consumption without the heat pump was 15 units.
The results for January show a peak consumption of 91 units (£9.94) in a day with an outdoor temperature range of 0 to 3C. The lowest consumption was 52 units with a temperature range of 9 to 11C. The overall cost was £208 in electricity saving £153 of oil – a loss of £55 and a total loss to date of £8,719 including loss of interest.
The results for February show a peak consumption of 78 units (£8.52) in a day with an outdoor temperature range of -2C to 4C. Typical consumption was 55 units at a cost of £6. At 10C the consumption was 43 units. The overall cost was £176 in electricity saving £153 of oil – a loss of £23 and a total loss to date of £8,773 including loss of interest.
The results for March show a peak consumption of 59 units (£87.26 - the cost of electricity increased to 12.31p per unit on the 1st) in a day falling to 16 units (£1.97) towards the end of the month. The overall cost was £148 in electricity saving £169 of oil – a gain of £21. A 24 hour temperature close to 10C used 34 units. On half the days the solar panels paid for the cost of hot water and on one day the cost of heating as well.
In the first half of April the units used fell from from 30 to 5 per day, but from the 19th onwards was zero, all the power for hot water being provided by the solar panels, and the high temperatures (day and night) meant there was no need for central heating. The heat pump was switched on for only an hour a day starting at 13:30 when the sun was still high and the air temperature was also high. The water temperature increases at the rate of 1C every three minutes, and the tank holds enough to last till the next day. The cost of elecitricity increased to 12.63p per unit on 1st April. The total loss to date is £8690 including loss of interest.
No electricity was imported in May for the heatpump so the cost of running it (only used for hot water) was zero saving £169 in the month and £73 to date. I estimate that the heat pump consumed about 93 units in the month so would have cost £11 to run without the solar panels. This indicates that the cost of producing hot water is about 30p per day on a hot summer's day and about 60p on a cold winter's day. This can be compared to the £1.60 a day we pay to buy the water.
We imported only 2 units in June. We have saved around £407 over oil since the system was installed. The total consumption of the heat pump is difficult to estimate accurately - it has used 4987 units of imported electricity and an additional 540 units of electricity we have generated giving a total of 5527 units which would have cost around £700 so even without the solar panels it is £300 cheaper than oil over 6 months.
In July we imported just one unit for the heat pump, and I estimate it used 93 units in total, giving a saving of £138 over oil.
The August figures are very similar to July. We imported just 4 units in the month, used 93 units in total, and a saving of £174 over oil because we would normally have had a new delivery in July and the price has gone up to £0.562 per litre.
We would have had the oil boiler serviced for £80 at the start of September - the heat pump does not require servicing so this is relected in the savings on the first of the month in the Spreadsheet. We imported 9 units of electricity costing £1.14 in the month and used a total of 90 units. We saved £247 compared to using oil bringing the total saving for the first nine months to £1003. The total imported electricity is 5001 units and used an additional 802 - a total of 5803 units or £732 without the solar panels and a saving of £837 over oil.
The central heating was switched on on 8 October - prior to that the heating has been off since 19 April, and the heat pump was only used for domestic hot water for which we imported a total of 16 units. I assume we used 3 units per day or about 540 units in the summer so subtracting 16 gives a saving of 524 units or £66. I guess that in October the domestic hot water uses 5 units a day and the rest was central heating. The heat pump imported 269 units which cost £33 in October saving £140 on the estimated cost of oil. Overall we have saved £1143 to date, but this reduces to £793 when loss of interest and inflation are taken into account. The heat pump is now left on all day with the thermostat set to 18C at night, 19C during the day and 20C in the evening.
In November the units used increased from about 20 per day at the start of the month to about 40 at the end. I assume that domestic hot water used about 6 units per day. The cost was £121 saving £168 on oil to give a saving of £47.
In December consumption was up to 1702 units. Over the year total consumption was 8364 units, but 473 units came from the solar panels so we paid £963 for the year. The estimated cost of oil for the same period is £2087 giving a saving of £1126. As we have solar panels the cost after taking the income into account was only £46. A breakdown by month can be found here, and a detailed breakdown by day here.
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