Waste heat from electricity generation could be a vast source of low- or zero-carbon heating for our cities, argues William Orchard
Is piped reject heat from electricity generation the most sustainable option for the UK? This byproduct is the UK’s largest untapped resource of low- and zero-CO2 heat. It was evaluated by government in the late 1970s and early 80s. But in recent White Papers, including the current Energy White Paper on nuclear power, city-wide combined heat and power (CHP) and piped heat supplies have not been considered.
Instead of these large systems, the government has chosen to focus on small and micro CHP systems such as a 1kW gas-powered domestic CHP unit and industrial CHP systems.
The potential of city-wide CHP systems that give the greatest CO2 savings have been all but ignored by the government (with one recent exception: the Office of Climate Change’s recent call for evidence on renewable sources of heating energy). This is a grave oversight because well-designed piped heat supplies in cities would halve the UK’s requirement for gas and hence double the time taken to exhaust the country’s gas reserves, reducing our national dependency on imported gas.
To understand the benefits of CHP you must first understand the principle of it. A car is a small CHP power station. In winter waste heat from the engine is piped to the heater to keep the occupants warm. The important thing is that this heat is delivered to the occupants at no extra cost for the journey and without burning any extra fuel, because it would otherwise have been wasted heating the environment.
This waste heat effectively has zero CO2. And, while it does not reduce the fuel for the journey, it does reduce the cost and CO2 footprint because the motorist would need to buy something else to keep warm even if that something was a hot water bottle, some warmer clothes or a miniature boiler!
In the same way that a car’s heater utilises waste heat from the engine to provide CO2-free heat, so cities can use waste heat from major power stations to provide very low- or zero-CO2 heat to all buildings.
DEFRA, in its October 2007 analysis of the UK potential for clean heat and power, estimates the potential for electricity generation from city-wide CHP systems is 33125MW. By using piped heat from this CHP, DEFRA estimates 159881GWh of energy can be saved a year. This level meets the UK government’s CO2 targets and all the UK electricity generation requirements without the need to build nuclear.
To get an idea of the possible savings in CO2 per household by using the waste heat from power generation, it is necessary to understand just how much CO2 households emit using conventional heating. The difference in the CO2 emissions for a flat is illustrated in Figure 1, which compares piped heat supply from three different CHP sources compared with heat from a gas boiler and from electric heating. The impact of insulation is also shown.
The important thing to note from Figure 1 is how the benefit of insulation changes as the heat supply system changes with the CO2 footprint of the energy source. Heat from electricity has four times the CO2 of heat from a new gas boiler and over 20 times the CO2 of waste heat from city-wide gas-fired generators.
An electrically heated flat, insulated to Part L of the Building Regulations, will have a far higher carbon footprint than an old uninsulated flat with an old gas boiler. Orchard and Partners London’s work on a project in the London Borough of Southwark showed the insulation of flats would have cost between £2000 and £3000 per kilowatt of heat load displaced for external insulation and double glazing.
Once a building is supplied with city-wide piped heat, the low marginal cost and low CO2 footprint of the CHP-derived heat will change the economic case for expensive insulation. As a result many conventional insulation measures, such as retrofitting insulation to the walls of pre-war domestic homes, are no longer viable.
There is a strong case to change the Building Regulations to remove requirements for minimum insulation levels and to allow designers to optimise their investment between low CO2 piped heat and insulation. The actual CO2 savings from piped heat depend on the fuel used for the CHP and the energy source and fuel for the alternatives (see Table 1).
Reduced dependency on gas
Another major benefit for consumers of piped heat is that it removes their dependency on gas (a virtually monopoly supplier to the heat sector) and they avoid costly and disruptive retrofit insulation measures. Orchard Partners London has identified a new, less disruptive and lower cost route to install piped heat supplies in cities by replacing kerb stones. This solution appears to be a practical, more sustainable, less disruptive, and less costly option for consumers than replacing gas pipes deep below the carriageway.
For the government, installation of piped heat could significantly reduce the carbon impact of new coal-fired power stations. On 2 January, the government gave permission for construction of Kingsnorth, the first coal-fired power station to be built in the UK for 24 years. If the waste heat from this power station was piped for use as heat, displacing the current mixture of heating and electricity from central gasfired combined cycle gas turbine and old gas domestic boilers, a city’s overall CO2 footprints would be similar. The CO2 savings would be even greater if the government chose to make Kingsnorth a nuclear power station. However, piped heat has not been evaluated in the White Paper on nuclear power.
Claus Hojlund Rasmussen, who has designed a number of the largest heat transmission systems in Denmark, has done a back-of-an-envelope calculation for a pipeline to serve London with heat from Sizewell nuclear power station 128km along the coast in Suffolk. A 2m diameter line would carry 2200MW of heat, leaving the power station at 95ºC and arriving in London at 95.1ºC after picking up pump energy as heat from the pipe friction. Offset against this heat would be the pump load at 54MW and the heat loss of 30MW. The CO2 footprint for this heat (assuming the electricity for pumping came from coal-fired power) would be 0.026kg CO2/kWh. Whether the cost of the line would be economic is another matter.
Unfortunately the government’s own statistics are compiled in such a way as to mitigate against signalling the benefit of using clean heat for the energy sector. The Digest of United Kingdom Energy Statistics (DUKES), produced for the Department for Business, Enterprise and Regulatory Reform (BERR), provides a detailed and comprehensive picture of energy production and use over the last five years.
Unfortunately, DUKES cannot be considered a sound basis for the analysis of CHP or for signalling the actual CO2 emissions of CHP on the respective heat and electricity sectors in our national statistics. This is because it considers that any fuel burn in CHP should be shared between the two different products, electricity and the heat, in a ratio of two to one on the basis that roughly one unit of fuel produces heat and roughly two units of fuel produce electricity, as explained in chapter 6.34 of its publication. This defies thermodynamic laws, as any motorist’s practical experience of using their car heater will demonstrate.
The result of the government’s method of analysis is plotted in Figure 2. The difference between the green line and blue line in Figure 2 illustrates the flaw in the DUKES analysis. By selecting the efficiency of your car at, say 25%, you will see that, with the DUKES methodology, simply by using the heater you should halve the fuel for your journey from 4 units to 2.
DUKES was used as part of the government’s energy modelling system in the recent Nuclear White Paper. The BERR carbon abatement curve must have used similar principles, as a saving is shown for micro CHP in both the heat and electricity sectors when in practice CHP can only give a saving in the heat sector.
A further major problem with government information is that it fails to present the savings from CHP, which can mislead decision makers.
The percentage savings of CO2 or fuel from the use of CHP are very different depending on whether you calculate the savings as a percentage of units of electricity added to units of heat (a combined saving across two quite different sectors), or the percentage saving for electricity and heat as separate entities and sectors.
For example: if a small amount of heat is taken from any power station’s cooling towers to heat a greenhouse, this would give a 100% saving in the heat sector because you would not need any fuel for heating. But if this system is presented as a saving for the electricity and heat sectors added together, heating a greenhouse from a 1000MW power station gives such a small percentage saving for the heat and electricity sectors the benefit is negligible.
This comparison of the methods is illustrated in Figure 3. The green and blue lines reflect the allocation to each sector in line with thermodynamic principles of the fuel burn for respective products. The orange shows line shows the saving across the two sectors added together.
Despite DUKES failings, the government has recognised the benefit of using reject heat and provides incentives to promote CHP. However, these incentives need improvement if potential CO2 benefits from CHP are to be realised.
The current incentive is on the production of electricity, which becomes exempt from the climate change levy. The incentive reflects the fact that there is an increase in the fuel burnt by a steam turbine serving a large city-wide scheme because the unit’s heat needs to be produced at a temperature high enough for space heating. However, since domestic consumers do not pay the climate change levy, suppliers of CHP heat and electricity over their own “pipe and wire systems” get no incentive under this system. This puts local authorities trying to utilise CHP at a severe disadvantage Opposition to city-wide CHP Despite its obvious cost, security of supply and environmental advantages, the piped heat supply option is opposed by the gas and electric utilities. Piped heat is not in their commercial interest since current rules mean both suppliers lose sales and revenue when piped heat is installed.
In the city of Odense, Denmark, piped gas supplies have been abandoned after the installation of a piped heat system. The large city-wide CHP system uses gas, coal or oil. Most houses and buildings are connected to the piped heat supply and, as a consequence, the buildings have had the lowest cost heat in Denmark for many years.
In a study funded by the Energy Saving Trust a few years ago, Orchard Partners London identified what it considers to be the optimum way to achieve UK CO2 targets by retrofitting cities with piped heat from local 500kWe electricity generators. These would be sited at each local electricity transformer supplying low voltage electricity and using their waste heat to serve about 500 houses. They could be designed in such a way that they continue to supply electricity and heat to residents even when central gas or electricity supplies fail. The current medium and high pressure gas pipes would be retained to serve the local 500kW CHPs, initially with natural gas and at some future date with biogas.
There is an excellent case now to benefit consumers with piped heat supplies: £5 billion of their money is to be spent replacing dilapidated local gas infrastructures. This money would be better spent if both electricity and gas utilities became city-wide heat supply utilities. This system would allow them to obtain better value from the fuel they import and to enter into interruptible and lower cost electricity and heat supply contracts, because they would have the ability to switch readily from one fuel to another to play the market as they do in Odense. Such a strategy will ensure a continuing role for utilities as heat suppliers when gas is no longer available or cannot be purchased at a reasonable price.
This solution will also be beneficial for all renewable energy generation, particularly wind. Numerous 500kW CHP units at every substation, generating electricity locally, free capacity in the high voltage transmission and distribution system. These local CHPs can change their operation: generating electricity at times when the wind does not blow and, when the wind blows too hard, converting this electricity to heat which can be stored.
Piped heat supplies have an enormous potential. The Office of Climate Change’s recent call for evidence is an encouraging sign that at last government is starting to recognise the importance of the heat sector and the role CIBSE can play in the development of piped heat supplies to cities. It is important they act fast piped heat systems could help the UK meet its electricity and CO2 commitments, without the need for expensive nuclear power.
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Building Sustainable Design
William Orchard is a director of consulting engineers Orchard Partners London. He was the first chairman of the Combined Heat and Power Association and acted as adviser to the Select Committee on Energy in the 1980s.
Original print headline: "Harness this heat" (Building Services Journal, March 2008)