Combining technologies

Meeting a significant percentage of a building's energy needs with microgeneration is usually best accomplished with a mixture of compatible technologies. Even a low technology approach might, at the very least, involve windcatchers, passive solar air and DHW heating and a rock bin thermal store. Over-complexity is a perceptible risk with a higher technology approach; nevertheless, phase change materials, ground source heat pumps, advanced active solar water heating, several forms of solar photovoltaic technology and cogeneration are all well-developed, proven options with readily available hardware. Medium-scale wind is promising, biomass can be a very practical choice when and where the right sort of biomass is available at the right price; and in any location fortunate to have a suitable stream or river close by, a small hydro can be the most effective option of all. As technologies develop, more options will become available, and microgeneration will become more than an ethical choice. However, selecting the most effective combination for a particular project is not straightforward. Initially, a thorough assessment of a site's potential is essential. Key questions would be:

• Will it be possible to orientate the building to maximise the area of any solar water heating or PV panels while simultaneously minimising unwanted solar gain?

• Is there room on the site outside the projected building's footprint for the more space-hungry types of microgeneration options (for example, storage and processing of biomass, ground mounted wind turbines, earth tubes, underground thermal stores)?

Site topology is important; a marked cross-fall can make passive solar water/space heating a realistic possibility, a site in a depression is unlikely to be a good location for a wind turbine. Similar limitations apply to a site surrounded by rugged terrain, trees, or tall buildings.

Data on the ground conditions below the site is also important for more than foundation design: the presence or absence of aquifers or flooded mine workings will influence decisions on space cooling; the depth to rock head will affect any choice of ground loops for ground source heating; the type and conductivity of the soil would determine the size of any such ground loop.

There may be potential assets just outside the site's boundaries, such as deep water that can provide cost-effective cooling, or an energy-rich river or stream that could be used for both water source heating and cooling and electricity generation. There may even be an old watermill site nearby that could be revived, or a water or sewage treatment plant whose outflow could also be harnessed to generate electricity. A search should always be made for local sources of biomass. Some local authorities would welcome the opportunity to dispose of horticultural waste and out-of-date food from supermarkets other than to landfill; local food and drink producers may even pay for their hydrocarbon-rich waste to be taken away. Dependence on processed biomass - such as wood pellets - should be avoided unless a local source is both available and long-term.

Williams Offshore Installation
Wind turbines and solar PV are an effective combination on this offshore installation (Reproduced by permission from Eagle Power)

No decisions on wind power should be made without a proper survey of the actual wind resource at the site. This takes time, preferably at least 12 months. A reliable estimate of both average wind speed and prevailing wind direction is also important if natural ventilation is to be contemplated. Solar resource should be checked over for at least six months, to monitor the effects of any seasonal shading from local features, including trees, buildings, hills and mountains. A similar survey of hydropower potential is also desirable when a prospective site is available. Aquifer and ground temperatures could also be monitored, to further refine any relevant calculations. And it is always a good idea not to take potable mains water supplies for granted. Evaporative forms of cooling should never be adopted without a cheap and reliable supply of mains water being available.

There should also be a thorough investigation to ascertain the attitude of the local electricity supplier towards microgeneration and grid connection (see Chapter 1) and whatever national and local financial incentives might be available. Grants and subsidies should never be relied upon to justify the long-term economics of a microgeneration package; nevertheless, they can make a project initially much more attractive. There is, however, a danger that government funding, rightly or wrongly, might overemphasise one particular technology - such as solar PV in Germany - distorting any analysis of the options for a particular project.

As mentioned in Chapter 1, the next and crucial stage, the calculation of a building's energy needs, is a complex exercise. Demand varies constantly throughout each 24 hours and throughout the seasons, and is likely to vary unpredictably over the building's design life as climate change kicks in. It should be remembered that global warming does not mean that all areas will become uniformly warmer. Climate change could mean milder, wetter winters and hotter, dryer summers; it could also produce localised cooling as ocean currents change their patterns. Early indications are that weather will become more extreme. For example, building designers may well have to rethink their assumptions on wind and snow loadings and make provision for heat waves longer and hotter than anything experienced in the last 100 years. Unseasonable flash flooding can threaten building services, even in areas traditionally immune to flooding.

Heat emitted by lighting, cooking, computers, and so on, is an important factor in the overall energy flow patterns within a building. There are countervailing trends here. Consumers, if surveyed, will claim to be demanding ever more efficient appliances, but are actually purchasing more, larger and more energy intensive items such as large flat-screen digital TVs, games consoles, whirlpool baths, power showers, hot tubs and domestic air conditioning. Office equipment continues to proliferate; the paperless office is even further over the horizon, colour printers are becoming the norm, vending machines and water coolers are standard items. On the other hand, low energy light emitting diodes (LEDs) might soon become standard fitments now that true white LEDs are available at realistic prices. Predicting what contribution waste heat from lighting and appliances will make to the building's space heating and cooling loads over the medium- to long-term is a chancy business.

Nevertheless, such predictions have to be made, and increasingly sophisticated tools are becoming available to enable at least the short-term picture to be clarified. Advanced software is available that can allow for energy flows in to and out of the building under a wide range of conditions. Some programs can check solar gain and wind potential against local weather records, although local wind data is usually not accurate enough to give really reliable results for wind turbines, where micrositing is so important (see Chapter 5). Allowance for climate change is more difficult: one option is to run the model for a location significantly warmer than the actual location and calculate the 'future-proofing' premium that this would involve. Thanks largely to a number of ' House of the Future' projects in different countries, more data is becoming available to validate the models. It is now possible to make reasonably accurate estimates of seasonal energy demands on most projects, and then to look at how some of this demand can be met by microgeneration.

It should always be remembered that there is no virtuous target, no ethical minimum percentage which separates a true low carbon building from a cynically greenwashed project. The target percentage of overall annual demand to be met by microgeneration might, for very good reasons, vary between 10 and 90% on individual projects - (100% if one includes fossil-fuelled back-up generators in the microgeneration equation). Much depends on the site, on the primary function of the building, and on the availability of grants and subsidies at the time of planning. For every project there will be a realistic optimum percentage beyond which the law of diminishing returns will set in.

This optimum will also be affected by the aspirations of the building's owners/tenants. Currently, the most common motive to go down the microgeneration road is probably the desire to insulate against instabilities in fossil fuel supply: to minimise the impact of potentially soaring prices and to cover for politically driven interruptions in supply. Maintaining at least some function when national energy grids are down could be a prime imperative for some.

Others might wish to be seen to be making an ethical commitment. This might be reflected in a preference for the high visibility of a wind turbine, for example, even when a wind turbine in practice may not be the most cost-effective solution in that particular location. In such cases, the wind turbine would have to be backed up by a less visible but more appropriate technology. Those clients who are really serious about achieving a low carbon footprint may also insist on materials with low embodied energy being used, which will restrict the use of some types of solar photovoltaic cells, for example. A far-sighted client may also wish consideration be given to the environmental implications of the eventual demolition of the building - some materials may pose a serious disposal/ recycling problem.

Most solar thermal equipment includes a high proportion of easily recyclable materials such as steel, copper, aluminium and glass. Polymer content is growing as metal prices soar - but so is polymer recycling capability. Similar comments apply to wind and hydro power. Fuel cells, PV arrays and most forms of battery are more problematical. More complex technologies - biomass gasification, anaerobic digestion, cogeneration and tri-generation - are relatively easy to dismantle and have a high content of material that could be recycled.

Every effort should be made to reduce overall energy needs before considering the specifics of microgeneration. High levels of insulation and the control of airflow into and out of the interior should be a feature of every modern building. Passive technologies should be evaluated at an early stage. Even the most aggressively advanced technology building can take advantage of passive solar gain: passive solar air heating can be integrated into advanced cladding systems, atria can function as effective solar chimneys. Windcatchers or windscoops can be striking architectural features. Well-sited pools and fountains can produce a more benign internal environment in more ways than one. Where there is significant cross-fall on the site, low-level solar collectors can function passively, a reliable and cost-effective water-heating option whose only significant drawback might be the potential risk from vandalism.

Passive Solar And Low Energy Strategies
This roof features solar PV, windcatchers and lightpipes (Reproduced with permission from Monodraught)

Moving up the complexity ladder one step might involve the use of motorised dampers to control natural ventilation, or solar PV-powered pumps or fans to drive water or air through solar collectors. Another upward step would take the designer to technologies such as evaporative cooling, water and/or air heating powered by biomass combustion, mini hydropower, wind and solar PV. Those seeking the ultimate solution would be considering biomass gasification or anaerobic digestion, trigeneration, unitised regenerative fuel cells and the like. On most major projects, however, a mixture of technologies from the simplest to the most complex will usually be the answer.

Where space permits, a separate 'powerhouse' for microgeneration has the advantage of imposing no architectural restrictions on the project proper. Vertical axis wind turbine manufacturer TMA (see Chapter 5) envisages the support structure for its turbine housing not just a generator and control equipment but also a biomass fuelled generator to act as standby. Similar principles could be achieved with other types of wind turbine. Concentrating solar thermal and tracking solar PV arrays are probably best located at ground level in most cases, and biomass gasification and anaerobic digestion would be difficult to include within the footprint of most buildings.

When a best estimate of the building's energy demands is available, the first topic to be addressed is usually the energy storage that can be provided. Most commonly the energy to be stored is heat; electrical energy is usually best ' stored' on the local grid, although

Solarcentury
'Powerhouses'need not be boring, as this installation at the ARC building in Hull demonstrates (Reproduced with permission from Solarcentury.com)

realistic alternatives are now coming on stream (see Chapter 15). The bigger the thermal store, the higher the proportion of the building's needs for space and DHW heating that can be met by alternative energy supplies. Ideally, surplus heat produced/collected in the warmer months is stored for use in winter. Seasonal thermal stores using water have to be relatively large; for example, for a typical dwelling to meet virtually all its annual space heating and DHW needs from solar water heating, the store would have to be at least 10 m3, implying a cubical tank more than 2.15 m on a side and weighing over 101. Hardly practical for a single dwelling - although the use of phase change materials would make the dimensions more realistic - but a central seasonal thermal store serving a number of dwellings would normally look much more attractive (see Chapter 18).

Large (capacity) thermal stores can be fed by a number of secondary sources, including heat recovered from attics, outgoing air and grey water, cooling air from PV panels, and the heat generated by speed controllers on wind and water turbines. Cogeneration is often only practical when a large thermal store is available. The ratio between heat and electric power production is fixed, so that at times when electricity demand is such that a surplus of heat is produced, this surplus can be transferred to the thermal store. And where there is no mutually beneficial relationship between the building owner and the local electricity supplier, surplus electricity can be converted to heat and stored as well. On larger projects it may well be that the ideal thermal store solution might be a combination of low temperature seasonal and high temperature diurnal stores, perhaps involving PCM.

PCMs can also be used to add virtual thermal mass to lightweight structures, opening up a number of heating and cooling strategies that would otherwise be unavailable. Conventional thermal mass in the form of concrete or structural steel can also be the right solution, especially where long span structures are preferred.

Some microgeneration technologies are obviously very compatible. Solar heating and solar PV are a near perfect fit, with the PV arrays providing the current when most needed to power the pumps and fans of an air- or water-based solar-heating system. A true solar roof or active cladding would incorporate both solar collectors and PV arrays, in a ratio determined by the relative energy needs of the building, or the desire for the PV arrays to provide enough back-up power to keep key IT elements functioning when local electricity grids go down. Both active cladding and solar roofs are now well-developed technologies that can provide energy as well as successfully keeping the weather out.

Where power is needed during the night hours a well-sited wind turbine is more appropriate than solar PV. Even then the power supply will be intermittent. Intermittency is a potential problem with many forms of renewable energy even if generous energy stores are provided. This is more likely to be a serious problem with electrical power, where long-term energy storage is still not a well-developed technology. If a reliable source of power other than that derived from fossil fuels is a priority, then the realistic options are cogeneration using some form of biomass or biomass derived gas, or mini hydro.

Mini hydro is demonstrably green, especially if a former watermill site is rejuvenated. Biomass can pose some complex ethical questions. The most convenient forms, such as pelletised waste wood and grains, will often have travelled considerable distances before use, and some will have required heavy dosages of agrochemicals. Biomass crops could usurp land previously devoted to food production. Efforts to maintain the current highly

Earth Centre Doncaster Passive Cooling
Several technologies were combined at the ill-fated Earth Centre, Doncaster, England (Reproduced with permission from Powertech Solar Ltd)

mechanised and personally mobile Western lifestyle by substituting biofuels produced from sugarcane and corn for liquid fossil fuels are already putting pressure on basic food prices. Even carbon-rich waste, which is currently a disposal problem rather than an asset, could soon become desirable. A commitment to biomass-based technologies should only be made in the knowledge that continuity and availability of supply can never be totally guaranteed.

Insulated Thermal Mass New England
A super-insulated high thermal mass design was chosen for the Brocks Hill Environment Centre near Leicester, England. Solar PV, flat plate collectors and a 29kW wind turbine supply energy (Reproduced with permission from Henderson-Scott Architects)

A commitment to minimise the energy required to cool the building and its occupants carries less risk. Over the longer term, for many buildings the cooling demand will be the most energy intensive. Luckily, several alternative and reasonably well-developed options which can reduce or even eliminate the need for traditional refrigerative technology exist. Even with relatively small energy inputs, natural cooling based on wind scoops and windcatchers and aided by sophisticated powered dampers can supply much of the space cooling needs of the most complex buildings (see Chapter 17). Many developers will still prefer to have the reassurance of refrigerative air conditioning; but passive and low energy active cooling techniques can significantly reduce the size, cost and energy consumption of the air-conditioning system.

It also makes sense to use solar power or air, ground or water source energy or waste, or recovered heat to preheat the feed water into a boiler; even if the main energy source is biomass. This will normally be via a heat exchanger in a high temperature thermal store. Preheating can allow the size of the water heating installation to be significantly reduced, with a beneficial knock-on effect on the size of any biomass drying and storing facilities.

As a general principle, the objective should always be that as little heat as possible should be allowed to go to waste. A cost benefit analysis is always going to be central to the process of determining just how far this quest should go. Heat or energy recovery ventilation is usually cost-effective where space heating and cooling and DHW are top priorities - such as for residential accommodation, hotels, hospitals and the like - but is harder to justify on economic grounds alone for buildings with low night-time occupancy. The same comment is even truer for heat recovery from grey water. A lot also depends on the sophistication and effectiveness of the building management system, and on how easily the occupants can override it.

Occupant behaviour is a key factor in electricity conservation as well. There have been several cases where it is alleged that pioneering high technology low-carbon buildings have failed to meet their original targets, partly because their occupants behaved in unexpected ways. For example, they switched lights back on after the building management system switched them off, or opened windows to increase perceived comfort levels inside, or brought in fan heaters to warm up rooms that had stood empty for some time in winter. And, even with the best intentions, people are going to leave equipment on standby for long periods unless there is a positive programme of regular checks - or until equipment manufacturers put limits on how long their products can be left idly consuming energy, most of which will only put extra loads on the cooling system.

Such limits will probably require government intervention on an international scale. Government legislation, particularly through local building codes and regulations, is also going to have an unpredictable effect on project design. A rapid upgrading of standards in response to particularly dramatic events can lead to recently completed buildings designed to older codes becoming less attractive to potential tenants - the furore over means of escape from tall buildings post 9/11 is a good example of that phenomenon.

Good intentions and a willingness to explore the possibilities offered by microgeneration technologies may not always be enough. Local planning authorities have to be compliant, or at least co-operative, neighbours and local amenity organisations may have to be placated. In November 2007 plans were announced for a £20 million package of technologies designed to dramatically reduce the carbon footprint of London's landmark Houses of Parliament, a grossly inefficient Victorian mock-Gothic edifice on the left bank of the River Thames. Some of the proposals were uncontroversial: a natural gas-fuelled trigen-eration plant in the basement, aquifer cooling, rainwater harvesting and partial secondary double-glazing. Other proposals brought the inevitable howls of protest from conservation and heritage bodies. Fifty ' tidal' turbines in the restricted river zone next to the building with small vertical axis wind turbines mounted above them were bad enough, it was said, but what really triggered the outrage was the proposed 1.65 MW, 35 m high horizontal axis wind turbine sited in Victoria Gardens, a small park on the riverbank next to Parliament. Given its central urban location, such a turbine would be little more than a very visible symbol of the government's commitment to the battle against climate change, one that could backfire badly if it spent most of its life motionless. The proposed small vertical wind turbines above the tidal turbines are unlikely to fare much better.

True low-carbon buildings with massively reduced energy demands will become the norm in a very few years. Today's designers now have the tools to get ahead of the game. A range of reliable technologies is available, which may be combined to produce the right result for a particular project. No great leap of faith is required.

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