by Dave Elliott
‘Heat is very difficult to decarbonise and no consensus is yet reached on the mix needed for the long term and you will have seen that from the various different reports on the subject.’ So said the then UK Minister of State for Energy, Baroness Neville-Rolfe, at the Heat Summit last December, with the next phase of the Renewable Heat Incentive (RHI) central to the agenda. There certainly are some competing options, including community-wide heat networks, green gas supply networks, biomass and solar home heating and domestic heat pumps powered by electricity.
By Dave Elliott
The current decarbonisation plan in the UK is to replace the use of fossil gas for domestic heating by the use of electricity. That may have been slowed by the abolition of the Zero Carbon Homes policy for new build, with its 2016 deadline, but there is still a strong push in that direction, with a ‘Near Zero Energy in Buildings’ concept and specific programmes for roll-out of technologies like electric heat pumps, apparently one of the government’s favoured heat supply options, along of course with insulation and improved building design to reduce demand. The latter makes sense, the former, well that is debatable.
By Dave Elliott
The German Environment Agency (UBA) has produced a comprehensive review of options for removing almost all (95%) greenhouse gas emission by 2050, based on the existing 80% renewables programme for electricity supply, but also looking at all the other sectors – including heating and transport. As I said in my coverage in an earlier post, that is pretty challenging. But it says it can be done. www.umweltbundesamt.de/publikationen/germany-2050-a-greenhouse-gas-neutral-country
By Dave Elliott
Heat pumps are seen as a clever way to get an energy upgrade, with the input energy driving a compression cycle, pumping heat collected from outside a building into radiators inside, like a fridge working in reverse. Most systems use heat from the air or from the ground, but there are also some water-source systems. For example there are large water-source heat pump schemes in Scandinavia, feeding heat to district heating networks. About 60% of the total energy input for Stockholm’s Central Network is provided by a district heating plant with six large heat pumps using the sea as a heat source. Warm surface water is taken during summer, while in winter, the water inlet is in 15m depth where the temperature is at constant +3°C. Helsinki in Finland also has large heat pump plant producing district heating with capacity of 90 MW, as well as cooling, with capacity of 60 MW, using heat from the sea and from wastewater led into the sea from a central wastewater treatment plant.
These are large projects, but a medium-scale system is being developed in the UK, using Mitsubishi’s Ecodan pump, which was voted the best new product or technology at the 2014 Climate Week Awards. It’s the first application of a system of its kind in the UK, and is backed Mike Spenser-Morris, a local developer and director of the Zero Carbon Partnership. The heat pump will use the Thames to provide hot water for radiators, showers and taps in nearly 150 homes and a 140-room hotel and conference centre at Kingston Heights in Richmond Park, cutting heating bills, it’s claimed, by up to 20%. It’s based on using water drawn from two metres below the surface of the Thames, where the ambient temperature, sustained by ambient heat from the sun, stays at around 8C to 10C all year round. A system of heat exchangers, pumps and condensers boost that to 45C. The electricity used to power the system is supplied by Ecotricity, which makes it zero carbon. According to a report in the Independent on Sunday, the system is thought to have cost about £2.5m, though this is for a ‘first of a kind’ project. The cost of future systems should be lower, and the Renewable Heat Incentive can offset supply costs.
Energy Secretary Ed Davey told the Independent on Sunday: ‘This is at a really early stage, but it is showing what is possible. You never have to buy any gas- there are upfront costs but relatively low running costs. I think this exemplifies that there are technological answers which will mean our reliance on gas in future decades can be reduced. Here you have over 100 homes, you have a hotel with nearly 200 bedrooms and a conference centre that won’t be using gas. It will be using renewable heat from the nearby River Thames. This is a fantastic development. My department is exploring the potential for this sort of water-source heat pump across the UK, so we’re going to map the whole of the UK for the potential’: www.independent.co.uk/environment/climate-change/exclusive-renewable-energy-from-rivers-and-lakes-could-replace-gas-in-homes-9210277.html
As the Independent noted, in theory, any body of water, including tidal rivers as well as standing water such as reservoirs and lakes, can be used as long as they are in the open and heated by the sun. The Government has a target of 4.5 million heat pumps across the UK, though most will be using heat from air or ground and will be small domestic units. Prof. David MacKay, until recently DECC’s chief scientific adviser, has described a combination of heat pumps and low carbon electricity as the future of building heating. However, as I’ve noted before, there are limits to the viability of small domestic systems: they make most sense in off gas-grid areas. Larger units, feeding district heating networks, are more efficient, and make more sense in urban areas, where there are large heat loads. Operation at the larger scale also make it easier to provide an effective maintenance regime, important for heat pumps, which need careful adjustment and servicing to maintain optimal performance. Otherwise the coefficient of performance (CoP), usually expected to be around 3, can fall dramatically. For example, in winter in damp cold countries like the UK, the external heat absorption pipes of air source heat pumps can develop a film of frost, reducing the heat flow. Without regular de-icing, the pump then has to work harder, potentially, in the extreme, reducing the CoP to perhaps 1 or less- making it less efficient than a simple one bar electric fire.
Moreover, large or small, the current type of heat pump run on electricity, and it’s been argued that the idea of shifting to heat pumps instead of gas for home heating on a national scale may be suboptimal, since using heat pumps run on mains electricity generated in large gas fired-plants, may be no more efficient than using gas direct in a domestic scale condensing boiler. It’s also argued that the wide-scale use of electric heat pumps is impractical, since the electricity network could not supply the large amount of power needed – the gas grid carries 4 time more energy than the power grid. It’s perhaps worth noting in this context that in the 1950’s, Southbank’s Festival Hall was heated by a large 7.5MW gas fired heat pump using the Thames as a heat source, although it seems it was taken out mainly as it produced too much heat: it was oversized www.architectsjournal.co.uk/home/rolls-royce-performance/181204.article#
There is now renewed interest in gas-fired absorption cycle heat pumps. They are less efficient than the electric motor driven compression-cycle variant, but gas is cheaper/kWh than electricity, much of which, after all, is made inefficiently by burning gas (and coal), so a 50% net fuel saving is claimed. At the World Renewable Energy Congress in London in August, Prof. Bob Critoph from the University of Warwick noted that there were now three domestic gas-fired systems on or very near to market (Robur, Vaillant, and Viessmann) with others under development. He proposed a mixed heating solution with both gas-fired and electric heat pumps, and also the use of hybrid electric heat pump-gas boiler systems, e.g. for older properties. He felt that the proposed mix, whilst not being the minimal emission route, was an affordable and pragmatic solution to domestic heating. There are of course other novel ideas, for example solar thermal fired absorption cycle heat pumps, which may have relevance even in the UK, with the combined air source/solar Solaris system claimed to be 25% more efficient than standard air-source electricity-powered units depending on location: www.uk-isri.org/case-studies/solaris and http://cordis.europa.eu/publication/rcn/16280_en.html
Whatever the heat and power source, are heat pumps the way ahead? Some say that large community scaled gas-fired combined heat and power (CHP) plants, with CoP equivalents of up to 20, are better in energy efficiency and carbon emission terms than heat pumps of any scale or type. That may be true at present, but, longer term, if electric heat pumps use green electricity, or gas fired heat pumps use green gas (biogas or stored gas produced using surplus wind/solar-derived power), then net emissions would be near zero. Although the same would be true for green gas fired CHP.
In the final analysis, given its high CoP, CHP seems to have the edge for the moment, but, in economic terms, the optimal systems choice may depend on the location and the size of the load. One of the largest gas-fired heat pump systems so far is the 140kW unit at Open University: http://www.modern-building-services.co.uk/news/archivestory.php/aid/9841/__65279;Ener-G_teams_up_boreholes_with_absorption_heat_pumps_.html
In some locations, large water sourced units may make sense, but large gas-fired units might have even wider applications. But then so may CHP, linked to district heating networks. However, to complicate matters further, it may not be a straight choice between CHP and heat pumps: e.g. a heat pump can be run using electricity from a CHP plant, while using the heat from the CHP plant as its heat source, thereby upgrading the heat output. Plenty of room for innovation! http://setis.ec.europa.eu/system/files/JRCDistrictheatingandcooling.pdf
by Dave Elliott
‘40% of Europe’s energy use and a third of the greenhouse gas emissions can be attributed to buildings and much of this relates to heating and cooling. For example in the UK, 38% of all CO2 emissions are related to space heating. These emissions can be avoided or significantly reduced through a combination of holistic design, integrated renewable energy and high efficiency.’ So says a report on Sustainable Heating and Cooling of Buildings from Leonardo Energy, the European Copper Institute’s think tank, offering a good analysis of the options for reducing buildings’ energy consumption and carbon emissions. (more…)
I have often been less than impressed by reports from the Royal Academy of Engineering (RAE) , which usually seems to take a conservative line on energy issues, but their new report on heating for buildings seems overall very well done, although with lapses. It makes the sensible point that we need to deal with the building envelop first, but also notes that most of the houses that will be lived in by 2050 have already been built, so we must look to remedial measures. It also notes that ‘Manchester isn’t Leipzig’, and looks at patterns of heating need and perceptions of comfort. It assumes we are talking about well insulated buildings, and familiar levels of comfort, and it reviews the energy supply options for supporting that.
It sets the scene by pointing out that ‘If space heating could be decoupled from water heating it would change the selection criteria for heating appliances and boilers. There would no longer be a need for the heating system (as opposed to the hot water system) to be on standby during summer months or to be capable of operating at a sufficiently high temperature to prevent Legionella developing in water systems. All domestic heating is currently thought of as low-grade heat requirement, but there is a case for distinguishing space heating as low grade and hot water as medium grade. A policy for heat should separate these two different uses’.
It looks at heat pumps as a possibility, but is not too convinced. ‘While the general reduction in carbon intensity of grid electricity makes the use of electric heating (direct or via a heat pump) more attractive, peak heat loads tend to coincide with peak electricity loads. There is, therefore, a significant likelihood of heating demand being met by high carbon electricity generation brought onto the system to meet peak loads over and above the capacity of low carbon generators’.
It goes on ‘Air source heat pumps have been rising in popularity for new build in the UK, but this is partly an effect of the way in which electrical energy is treated in the regulations that makes CO2 targets more lenient than for gas systems.’ While it admits that ‘Air source heat pumps integrate well with well insulated dwellings, if properly sized and installed,’ and it suggests that ‘micro-CHP complements and could balance some of the properties of heat pumps’, it also notes that ‘several reports discuss inadequacies of the application or system engineering in heat pump installations. It is clear that heat pumps are not forgiving if installed inappropriately.’
By contrast, it’s much happier will larger-scale communal system. ‘Communal air source heat pumps are an interesting area of development with some new configurations of systems coming to market. Central systems may be more efficient and are likely to offer much greater energy storage than do systems designed for individual household’.
It adds ‘Larger district systems, incorporating a CHP facility and providing heating are significantly more efficient than domestic level installations. This is because waste heat can be used in district heating after it has generated an element of electricity. Such district heat is therefore always of significantly lower CO2 emissions than any heat only production utilising the same fuel’. And that, it seems, includes domestic scale heat pumps.
The RAE does seem to been moving towards community- scaled system across the board. However, it is less happy with renewables. Although it sees some potential for bioenergy e.g for CHP/District Heating , it is not very impressed with solar, and overall treats renewables as problematic in terms of grid power supply, reverting to the traditional RAE line on the problems of intermittency and the delights of nuclear: ‘During the summer months, most of the night-time load could be provided by nuclear power with renewables providing additional power during the day’, while in winter ‘we would need sufficient renewables to guarantee 40GW during the evening peak. As wind, tides and the sun are intermittent, that would require significantly higher installed capacity of renewables or thermal back-up capacity, much of which would be unused for long periods in the summer’ making renewables uneconomic.
Nevertheless, it does look at smart grid /load management options which might change the situation radically, helping to deal with intermittency. A bit grudging, but at least there is now some recognition that a new interactive supply and demand system might be viable. It’s taken decades to get the community CHP/DH message across to the traditionalist engineers, so maybe it’s too early for idea of smart dynamic grids to have got through! And it may take even longer for them to give up on ‘baseload’ nuclear, which they still see as essential, rather than as getting in the way of a more interactive flexible system based on renewables ( which is the view emerging from Germany) .
However, as far as CHP/DH in concerned, the RAE is now full of praise. It says that ‘CHP plants, biomass combustion, and heat pumps are more efficient, reliable and cheaper at scales larger than a single dwelling. The costs of large scale heat pump installations per kW are a quarter of that for domestic-scale installations.’ It adds that ‘it is more efficient to use the available skills for fewer large systems than for many individual units’, and that, since energy storage will be needed ‘district heating systems have another important benefit – the mass of water in the underground pipes provides a heat store that evens out daily peaks and troughs in demand. This can be supplemented by hot water tanks to increase energy storage’. And taking it one step further, it points out that ‘well insulated hot water tanks or underground inter-seasonal thermal stores will be simpler to provide on a community basis given the small (and reducing) size of most UK homes’.
Some sense last! And DECC seem to be taking notice, in their new Heat strategy- see my next Blog.
‘Heat: degrees of comfort?’ Royal Academy of Engineering www.raeng.org.uk/heat