Heat pumps could cut our local CO2 emissions by 50%, reduce the cost of domestic heating and end the problem of fuel poverty in high rise flats.
Heat normally flows passively from warmer things to cooler things. To make it go the other way we need a heat pump, which uses some external source of energy to drive heat "uphill" from cooler places to warmer ones. Heat pumps are not new. They were invented in the 1850’s and they have been used inside domestic and commercial refrigerators ever since. Air conditioning units and high-quality tumble driers also use heat pumps.
The original idea is very simple and still in use today. A mechanical pump is used to compress some harmless gas, thereby raising its temperature. This is the "hot" side of the pump, where heat is released, allowing the hot gas to cool towards its uncompressed temperature. The cooler, but still compressed gas is chanelled through a small hole. The pressure drops, the gas expands and cools below its starting temperature. This is the "cold" side of the pump, where heat is taken up, warming the gas towards its initial temperature. The gas then re-enters the compressor and the whole cycle repeats indefinitely. Heat is continuously taken up on the cold side of the pump and released on the hot side for as long as we supply external energy to drive the compressor.
There are numerous alternative designs for modern heat pumps. Some are entirely solid-state, without moving parts, but the most efficient still use gaseous refrigerants which liquify easily under pressure. Some popular refrigerants were found to damage the ozone layer, and have been phased out. Many more are powerful greenhouse gases, so it is important to prevent leaks. Natural refrigerants are available without these disadvantages.
The refrigeration concept is easily turned on its head and heat pumps have been used for domestic heating since the 1940’s. Today, heat pumps can reduce fuel poverty and help to combat climate change.
Heat pumps used for domestic heating come in three flavours, depending on their source of heat. Air-source heat pumps are cheapest and draw their heat from the air. Ground-source heat pumps draw heat from the ground, and are more expensive because this requires engineering works. Water-source heat pumps draw their heat from lakes, rivers or sea. This is the best option if suitable water bodies are available.
Electric heat pumps are typically four times more efficient than ordinary electric heaters, because it is easier move heat from one place to another, instead of creating new heat from scratch. Under current UK policies gas is much cheaper than electricity. It is therefore economical to use combined heat and power (CHP) units to generate electricity locally. This electricity is then used to drive water source heat pumps (WSHP) which feed hot water to consumers via a local heat network beneath the streets.
CHP captures the "waste heat" from electricity generation and distributes this to consumers via the heat network. Electrical energy used by the compressor is also converted into heat. This is captured too: nothing is wasted. The overall process doubles the heat available from conventional gas boilers and halves our CO2 emissions and resource consumption. It could end fuel poverty in high-rise flats, where gas may not be safe. CHP is an effective low-cost “add-on” for natural gas, bio-fuels and energy from waste, but offers no benefit with hydro-electric, tidal, waves, wind or solar PV. The current proposal works with existing buildings, provides local operating experience and creates vital network infrastructure that will be essential for full decarbonisation in the future.
Arithmetic: Here are the basic calculations behind this proposal. A typical water-source heat pump has a coefficient of performance (COP) of 4. This means that for each 1 kWh electrical input we get 4 kWh of heat out, comprising 3 kWh pumped heat, plus 1 kWh “waste” heat from the motor, compressor etc. Both sources heat the output. Electrical energy for the pump is generated locally from a CHP system with a typical efficiency of 33%, which means that each 3 kWh energy input from natural gas yields 1 kWh electrical output (which drives the pump) and 2 kWh of “waste” heat, which is captured. Adding both systems together, the overall result is that each 3 kWh gas energy input generates 3 kWh pumped heat output and 3 kWh “waste” heat output, all of which is used. Adding the heat pump doubles the heating power of natural gas.
Central Government: Since 2014 the Department for Energy and Climate Change (DECC) has identified the River Aire through Leeds as a prime source of pumped heat, piped to local homes and businesses through a buried heat network. The river continuously brings warmth into the area. Water has a high thermal capacity and transfers heat easily. River temperature varies less than the air. These factors imply that water-sourced heat will always be cheaper than ground-sourced heat, and more reliable than air-sourced heat. Subject to capacity limitations, these are permanent advantages.
DECC calculated that the River Aire through Leeds could supply 18.8MW heat for a temperature drop of 0.93°C. This corresponds to 165 GWh of heat per year. Electricity consumption in Kirkstall is about 50 GWh per year and natural gas use is about 152 GWh per year, so water source heat pumps could make a significant contribution to our sustainable energy supply. If the heat pumps were driven by combined heat and power systems, then (in addition to the pumped heat) a similar amount of waste heat could be captured and put to use. It appears that CHP + WSHP could supply the bulk of the energy to large areas (including high-rise) along the river valley in Armley, Bramley and Kirkstall, delivering major reductions both in CO2 emissions and in the cost of heat.
Homes account for 27% of UK energy consumption. Most domestic heating uses gas. We must eventually learn to leave fossil fuels in the ground, but in the medium term, in West Leeds, a local switch to CHP+WSHP from the River Aire could largely abolish fuel poverty and cut domestic CO2 generation by half. The UK is near the bottom of the European league table for heat pumps, saving only 0.4 Mt of CO2 in 2013 compared with 5.7 Mt in France. Major gains are therefore possible.
West Leeds is an excellent location for a pilot / demonstration plant because the Council owns land and buildings straddling Abbey Mill goit, which is a controlled watercourse, separate from the main river, but capable of carrying a substantial fraction of the total flow. Other suitable locations include Armley Mills and Kirkstall Forge, but central Kirkstall is the best place to start. Large, non-domestic “anchor” users surround these proposed sites: 27 GWh near Cardigan Fields, 17 GWh in Central Kirkstall, and 10 GWh near Kirkstall Forge. New developments, expected at Kirkstall Forge and on the former “Tesco” site in Kirkstall District Centre, would add to these totals.
Sources of information: The following websites are generally reliable, despite some minor errors. They are arranged in a logical sequence to illustrate current best practice. The detailed situation in West Leeds differs from previous schemes so we must eventually develop our own bespoke plan.
Water source heat pumps could be located at our four historic water mills: Armley Mills, St Ann’s Mills, Abbey Mills and Kirkstall Forge. All four locations provide an opportunity to work safely within regulated side channels, rather than the main river. The river flow varies enormously from day to day, from 4 cumecs (cubic metres / second) during a summer drought to 400 cumecs during a major flood. If the equipment were located in the main river it must withstand 400 tons of water per second landing on top of it, but in the mill goit the penstocks at Kirkstall Abbey (for example) can be opened or closed to provide a constant flow.
The maximum number of customers depends on the worst case assumptions. Maximum heat demand per household is normally during winter breakfasts, when the central heating has just started, and there is someone in the shower. The rule of thumb is 20kW with a gas combi boiler, but this is probably an over-estimate because over-sized gas boilers add little to the price. Morning showers are often very quick!
12kW heat pump capacity per household would be reasonable, but these will not run at exactly the same time. A recent study by Strathclyde University put the sustained morning heat load about 5kW per household on a winter weekday, if we all shower at slightly different times.
Heat pumps will cool the river. How much river cooling should be allowed? There is a strong argument for 2°C, on the basis that this would exactly compensate for the anticipated global warming, so our average river temperature would actually remain close to pre-industrial levels! The minimum flow in winter is about 6 cumecs, so the worst case pumped heat available is
2 x 4.2 x 6 = 50.4MW (yes, megawatts!)
Heat recovered from the CHP generator and heat pump motors will double this amount, so the total heat available is 100.8MW, sufficient for about 20,160 households. On a cold winter morning, two thirds of the families in Armley, Bramley and Kirkstall could be supplied with half-price heat from the River Aire!
The average mid-winter flow is about 25 cumecs and prolonged winter droughts are rare events. The heating consequences would be benign. We would just have to drive the CHP a bit harder. We could still meet the heat demand, at at significantly greater cost, by topping-up with full-price gas.
In practice it would be better to have a mix of domestic and commercial "anchor" users, to provide an adequate base load when the new plant is commissioned. That is exactly what we have now, with the Kirkstall shopping centre, swimming pool and brewery flats already in place, and a major re-development on the vacant "Tesco" site expected shortly.
This is a work in progress. These are published indicative prices, not necessarily the best price or good value for money. Our objective is an approximate estimate to supply 400 households (or equivalent commercial space) for scoping purposes. There is no cooling / air conditioning in these proposals. A practical system is likely to include multiple units with an intermittent duty cycle for maintenance. DECC also recommend several small installations rather than one big site for improved resilience and reduced environmental impact. It is therfore envisaged that all four historic water powered sites (Armley, St Ann’s, Abbey and Kirkstall Forge) will eventually be inter-connected and brought into use. All four sites will have many other concurrent uses, including Industrial Museum, Visitor Centre for the Kirkstall Nature Reserve, Library and Community Hub.
CHP generator: This is manufacturer’s data for LVHUAN 400kW gas generator with heat recovery taken from their website. The list price is US$ 40,000 – 50,000 each (£30,000 – 37,500 if 1US$=75p) This generator weighs 12.5 tonnes. The price does not include shipping, installation and commissioning.
This particular generator set requires >40% methane. Others will run on a wider range of fuels, so locally grown biomass could contribute to the energy input. Some annual energy crops could be incorporated into the crop rotation cycle on local agricultural land, but are unlikely to make a major contribution. The perennial Miscanthus yields about 63MWh/hectare/year but is difficult to convert the dried grass into a liquid or gaseous fuel on a small scale. Bioethanol from beet or potatoes is easier to produce and handle but the yields are lower, 33MWh/hectare/year. For comparison, one week’s full output from a 400kW generator requires over 200MWh energy input.
Heat pumps: Star refrigeration quote a guideline price of £500 per kW for their Neatpumps, so 400kW should cost about £200,000.
Water pumps: Assume that all the heat is transmitted to customers by hot water at its final delivery temperature running through insulated pipes. (There may be much better options.) Flow temperature 60°C, return temperature 40°C, 2.4MW thermal load. The required flow rate is about 100 m3/hr. Suitable pumps are available from Chinese suppliers for under $1000.
Heat Network: This likely to be the major cost. We can price this for a hot water system, but better solutions may be possible. Watch this space!
Last updated 29 August 2017 at 15:05. Back to the top
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