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Urban Renewables

Currently around 70% of the UK population lives in urban areas and, in effect, rely on rural resources for many of their basic necessities- most obviously food, but also water and energy. So, in effect, cities are dependants- even parasitic.

To put it in contemporary terms, their ecological footprint, that is the area of land that would be needed to support them and deal with their wastes, is many times the land areas they actually cover. For example the footprint for London is about 125 times the area of the city, and equivalent to nearly all the UK’s productive land area.

In this paper, we sketch out the lines of the emerging ‘sustainable city’ vision, focusing on energy. My emphasis will be on energy use in buildings, which is responsible for around half the UK’s current carbon dioxide emissions. In particular we will be focussing on energy supply- on the assumption that the crucial energy conservation and energy efficiency aspects will have already been dealt with.

Renewables in the City

It is now sometimes argued that, far from being a major energy drain, cities could meet a substantial part of their energy needs from their own energy resources, without adding to the problem of Climate Change, by using renewable energy sources. In the past, energy has usually been brought in to cities, from remote and usually polluting power stations, by wire or pipe. However, there are now technologies available that can use clean renewable energy sources which are directly available in cities, solar photovoltaic (PV) technology being the most obvious. Cities actually offer an ideal location for the installation of solar PV modules. For example it has been estimated that, if PV arrays were mounted on suitable south facing roof tops and facades in a city like London, they would have a total generating capacity similar to that of around two conventional power stations.

Of course, there are drawbacks with solar PV. Solar energy is not available continuously, especially not at night! Never the less, some types of urban energy use are well matched to solar energy availability, most obviously day time office electrical loads and, in particular, summer time air conditioning loads. In addition, if power from PV arrays is fed into national power grid, low levels of solar energy availability in one location can to some extent be compensated by excess solar energy available in other areas, with the grid acting as a buffer, balancing out local variations

However, if power from PV arrays can be stored in some way, then of course, the solar options widen. The most obvious storage route is to use electricity from PV arrays to electrolyse water to generate hydrogen gas.

Although safety controls have to be stringent, hydrogen can be stored and transmitted down pipes and used where and when needed, either for direct heating, as a combustible fuel, or in a fuel cell, to generate electricity. It can also be used as a fuel for vehicles, either directly or via a fuel cell.

PV is of course still expensive. Currently PV arrays on a typical domestic roof costs something like 20,000 and will only offer a few kW of generating capacity. Enough, on average over the year, for lights and most other domestic electrical systems, but not enough for major loads like heating. However, usually such systems are linked to the grid and extra can be imported when needed, with any excess power being exported, thus potentially reducing overall electricity bills. Sadly, at present the electricity companies typically only pay as little as 2p/kWh for power sold to them and charge up to 7p/kWh for power bought from them. So PV is not very attractive to many people.

However, matters should improve. Firstly, PV cells are getting cheaper. As and when demand for them rises, their price should drop dramatically. One study suggested that given the creation of a reasonable market, PV would become competitive with conventional sources. Secondly, there is pressure on electricity companies to accept the idea of ‘net metering’. Consumers who are able to export some power themselves would then only be charged, at reasonable rates, for the net amount of power transferred. One company (Eastern, now part of TXU Europe) is now offering this option. In addition, the full VAT charge has now been removed on professionally installed solar systems, including PV, so there should be more of an incentive to invest in this technology.

At present however, those people who have installed PV arrays on their houses in the UK, such Jeremy Leggett in Kew (retrofitted) and Susan Roaf in Oxford (newbuild), have done it as a demonstration of what can be done technically, rather than as an economically attractive investment. Never the less they do report that their electricity bills are now very small if not at times negative, and Sue Roaf runs a small electric vehicle from her house power.

The economics of PV also need to be set in a wider context. A modern integrated PV roof, made up of solar PV tiles, can substitute for an ordinary roof, so some of the cost of the PV system is offset, and this cost will in any case be offset by the value of the power generated. What other building item will actually earn it’s keep? This same argument concerning costs has also been used on a wider scale. Prestige corporate headquarters office buildings often have vastly expensive facades of marble. A PV array could add the same glamour at around the same cost, but also generate power.

So far I've focused on PV solar for electricity generation, but there are other urban solar options, most obviously direct solar heat collection, for space and/or water heating. London has around 200 solar house projects, many of them the legacy of the old GLC/ South London Consortium days, and there are some 40,000 water heating systems installed around the UK. Passive solar design has subsequently become common in many new buildings, as a cost effective design option. Typically this can cut fuel bills by up to 30% over an average year.

In addition there are of course a whole host of other ways to improve the energy efficiency of buildings, by good design of the body shell, proper insulation levels and the use of low energy appliances and domestic systems. The potential for energy saving is considerable, and as buildings become more energy efficient, then it becomes easier to supply their reduced energy needs from renewable sources. Direct solar energy is of course only one of the renewable energy options available in cities. Cities also generate huge amounts of domestic and commercial wastes, which have a high calorific value, most of which is derived ultimately from solar energy. The first step should of course be to try to reduce the amount of waste produced, at source and then by composting, and recycling, but even so, cities are still likely to produce large amounts of waste, and it seems sensible to try to recover the energy from the non-recyclable waste that is left.

Recovering energy via waste combustion is not always popular, given fears about toxic emissions, and there are limits to the number of landfill sites we can accommodate near to cities, even if the methane gas they produce is tapped and used to generate power. Fears about dioxin emissions from poorly run incineration plants continue to plague waste combustion projects ( see Renew 130). One possibility is to move to pyrolysis, which seems to have fewer problems. Even less problematic is the well established technology of sewage gas combustion, and sewage is one resource cites have in plenty.

Of course all combustion processes inevitably generate carbon dioxide, a ‘greenhouse’ gas which contributes to Climate Change. However, recovering energy by waste combustion is at least partly greenhouse neutral, in that most of the original material is biological, and carbon dioxide was absorbed in its production. As for landfill gas, while there can be problems with toxic leachates, since we do have landfill sites, it is surely better to capture methane from them, rather than letting it escape into the atmosphere. It is a very powerful greenhouse gas.

Whether or not you view it as strictly renewable, the use of domestic and commercial wastes as a fuel is currently very commercially attractive. It becomes even more interesting if it is linked with co-generation- that is using the heat produced as well as the electricity, in Combined Heat and Power (CHP) Plants. They can more than double the overall efficiency of energy use, and small local CHP plants can feed heat to local district heating networks. But CHP and district heating really comes in to its own if use can be made of natural biomass as a fuel rather than wastes. Cities may not seem like an obvious source of biomass, but most cites do generate surprising amount of wood wastes from parks and the like, and some energy crops might be grown on brownfield sites. Some modern low density cities, like Milton Keynes, should have sufficient woodland and parkland wastes to run sizeable power plants. We are working on a wood waste fired system for the OU.

CHP plants do not have to be large. Whereas in the past there have been plans for city wide CHP/district heating systems, these days the emphasis is on smaller units meeting local heat loads. Indeed some people even think we could move to domestic scale units, with these eventually being run off renewably-produced hydrogen gas, piped down a gas main, but with natural gas being used in the interim.

The bottom line, in this quick sketch of the main urban energy options, is that, given sensible attention to building design and the efficient use of energy in low energy devices and appliances, energy demand could be cut significantly and renewables could make a significant contribution. For example, studies of Leicester carried out by the OU Energy and Environment Research Unit, suggested that demand could be cut by at least 60%, and much of this reduced energy requirement could then be met by CHP plants and from locally available renewable sources, with renewables supplying perhaps around 25% of the cities power. As a result, by 2020, Leicester’s C02 emissions could be cut by 80% on 1990 levels.

However, this analysis does not exhaust the possibilities for urban renewables. In some cities, there are also other opportunities for using renewable sources, for example, micro-hydro projects on small rivers and even streams. One such project is underway on the River Wandle in S. London. Another option available in some locations is geothermal energy. Although not strictly renewable, heat can be extracted from aquifers and fed to district heating networks, as has been done in central Southampton.

More generally, and without geographic constraints, use can be made of ground source heat pumps for heating buildings. By contrast, the prospects for the use of locally generated wave and tidal power directly in cites are limited, since the main wave resource is in remote areas offshore and few cites are on rivers with significant tidal ranges. However, in some locations it might be possible to use power from small onshore wave energy units or tidal stream generators in estuaries.

Finally, to complete our review of novel renewable sources, what about wind power? So far, wind power has been seen as pretty much irrelevant in urban contexts. Wind speeds are usually too low and finding acceptable sites almost impossible. However, a colleague at the OU, Dr Derek Taylor, is developing a novel building integrated wind turbine system which fits into the roof space of a house, the Aeolian roof. This takes advantage of the fact that wind speeds can be increased by appropriately shaped roof designs. Assuming that noise and vibration problems can be avoided, we may yet see wind power make an urban contribution .

Including wind power with the other novel renewable energy sources I have outlined above, it may very well be possible that, in some situations, the contribution from urban renewable energy sources could rise well beyond the 25% suggested above. Moreover, assuming continued improvements in the efficiency of energy use in buildings and elsewhere, and the integration of energy efficiency and energy conservation systems with advanced renewable technologies, such as renewably powered hydrogen fuel cell systems, then the urban renewables contribution could rise much higher.

However, total urban self-sufficiency seems unlikely. So, the rest of the power required would have to be imported from rural areas - from remote windfarms, hydroelectric projects, offshore wave energy devices, tidal stream devices and so on. Some of the energy from these non-urban sources would of course be best used locally, and that is especially true of locally grown energy crops. But there should be sufficient extra, in time, to supply the cities.

Of course not all rural populations will relish having wind farms and energy crops plantations imposed on them just to service the energy needs of urban areas. That’s one reason why it is important for cities to try to resolve their energy needs themselves. It is true that PV solar is still in its infancy, at least in the UK, but this option, along with the other renewable energy options I have mentioned, offers a chance to deal with the urban energy problem at source.

The bottom line is that, if properly designed, with proper attention to energy losses and efficient energy use, many urban buildings can use renewables to meet a significant part of their remaining energy needs. What we have to do now is make it happen both in the UK and, of course, around the world.


* This is an edited and revised version of parts of a paper presented by Dave Elliott to the UK ISES Conference on Building for Sustainable Development at RIBA last year. This version appeared in Renew 130

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