More sustainability terms explained in plain English, courtesy of Turner & Townsend
We continue to hear about sustainability and renewable energy practices every day. They arrive at such a rate, it can be difficult to keep abreast of every process. We have accessed this guide to help explain the fundamentals of some of these terms and techniques. We would once again point out that they cover only elementary detail, but it should be enough to get by at a superficial level.
Micro-generation is the small scale generation of low and zero carbon (LZC) energy or heat, power and domestic hot water for use in the home, small business and communities buildings to meet their own needs at the source of consuming the energy. It includes renewable energy such as wind turbines, water turbines, ground source heat pumps, solar thermal collectors, photovoltaic cells (pv) electricity; Micro combined heat and power (CHP) and biomass heating systems mainly in the form of log burning stoves, with or without back boilers.
Combined Heat and Power (CHP)
Combined Heat and Power (CHP) does what it says on the label – combines heat and power generation - also known as cogeneration. In similar mode to a power station it combines a boiler producing a gas to drive a turbine which generates electricity, a significant bi-product of chps is very ample quantities of hot water which be used for heating DHW and for cooling – see below. As such it’s the most efficient use of all fuel types. While a conventional power station dissipates over 60% of the heat created out into the atmosphere through coling towers, or in the case of nuclear power stations to the sea; a CHP uses the waste heat and is therefore virtually 100% efficient. Scandinavia, the Netherlands and Germany are testament to the popularity and environmental benefits of state-of-the-art CHP technology that is now available, especialy when deployed on a community wide scale.
The Government has set demanding targets for CHP in their 2016 timeframe and with pump prime funding available for the public sector through the ‘Community Energy Scheme’.
An additional advantage of chp is that the ample hot water supply can be used in a ‘tri-generation’ scenario to provide cooling through a chemical absorption process, which is exceedingly efficient.
Biomass, Biofuels and Biogas
In the UK these are still regarded as the Cinderella of renewable energy, close at hand but never quite invited to the ball, however they are the moist sustainable of all fuels, if occasionally somewhat smelly. Biomass is any cellulose material in the shape of wood chippings, pellets, waste wood or even straw in briquette format. Short rotation coppicing such as willow and hassle of are highly sustainable biomass fuels. The advantage biomass is that it is 100% sustainable nature if harvested from sustainable forests through what is known as the sequestration process. Trees during their growing cycle release oxygen and absorb carbon dioxide they are then felled and replanted and the process starts all over again. A wood burning stove fitted with a back boiler is a micro biomass boiler.
Biofuels come from harvested crops such as rape seed oil and maize. However waste cooking oil and can also be converted into a biofuel
Biogases in the form of methane is a by-product of animal waste or from the fermentation of vegatable matter which can produce an ignitable gas. Sewage systems which also produce methane are another source of biogas which in the UK are currently untapped.
Waste to Energy
The black sheep of the renewable energy market past waste to energy projects like the West London waste centre had a poor reputation due to their toxic emissions. Domestic rubbish is used as a fuel by burning it at very high temperatures. Such systems require careful engineering to ensure that the toxins in the fuel gases do not escape into the atmosphere. However if properly engineered waste to energy is a highly sustainable solution to the nations domestic waste burden, even more so with the yearly reduction of land fill sites which by 2013 will have virtually disappeared. Several schemes by a number of local government and public bodies are presently being considered in terms of both a renewable energy solution for community chp plants and as a long term answer to managing waste.
Ground Source Heating and Cooling Pumps (GSHP)
At about 1.0m below the ground level the ambient temperature is stable at about 50C and this can be harnessed using GSHP. GSHP systems have four essential components:
- A pump
- A condenser
- An evaporator, and condenser
- Plastic pipes known as ground loops or slinkies laid at a depth of 1.0m
Operating on the same principle as a fridge, ground loops or slinkies contain a water based refrigerant or ‘brine,’ which absorbs the latent heat of the ground. The refrigerant is then pumped through the evaporator and condenser under pressure to raise the temperature to approximately 500C to heat water for distribution around a building, normally in an underfloor heating system. Although providing sufficient hot water for heating an average 3 bedroom house, they would not be able to meet all the DHW needs of a domestic hot water cylinder, consequently most GSHP package today are sold with roof mounted solar thermal hot water heating systems – see below. Powered by electricity GSHP is not strictly providing renewable energy, however due to their high ‘coefficient of performance’ ratios of power in to heat out they have fairly low energy consumption and associated CO2 emissions when compared with conventional gas fired domestic heating systems and can showing savings in the order of 40%.
Larger commercial system using deep well bores harness the natural geo-thermal properties of ground water at approximately 100m deep, operating on the same principle as domestic scale GSHP, but on a larger scale.
Returning to the fridge principle, the GSHP process can be reversed to provide cooling, although air-source pumps are more efficient in this situation, nevertheless the basic mechanics are similar.
Wind Turbines – big and small
Wind turbines range in size from 1KW to 20MW, using the natural power of the wind’s energy to power rotor blades that turn a turbine and create electricity, at relatively low speeds, on the same principle of a traditional wind mill. Traditionally wind turbines are of the upwind type where the wind drives the rotor blades face on. But recent technology developments in the design of rotor blades has seen vertical axis rotor blades come on to the market. Whereas traditional upwind blades require high start up speeds, vertical rotor blades can operate at very low wind speeds of around of around 2-3m/sec compared with 4-6m/sec for an upwind system. Furthermore upwind systems are very location sensitive. A clear wind path of approximately 100m radius in all directions for them to operate efficiently, which precludes their effective use in urban areas, or come to that coastal locations where the wind might be variable in direction, known as ‘dirty wind’ to the sailing fraternity.
A wind farm is a collection of wind turbines in the same location feeding wind-powered electricity into the National Grid. Currently in the United Kingdom there are approximately 145 on and off-shore wind farms with more going through the planning process as the table below:
Projects - 145
Turbines - 1839
Megawatts - 2157.125
Homes - 1206154
Equivalent CO2 reductions (pa) - 4875275 tonnes
(From BWEA Wind data see http://www.bwea.com/ukwed/ )
The United Kingdom has the best potential for wind energy in Europe, hence the Government's eagerness to see its development as a large scale renewable energy source.
Photovoltaic Cells (PV)
PV absorb daylight (contrary to general opinion - sunshine is not necessary) merely radiation and converts it into electricity. Photovoltaic cells consist of two or more thin layers of a semi-conductor material, usually silicon, and designed with a positive and negative layer to create an electrical field. As with wind turbine the electricity created is in DC format and has to be converted to AC via an inverter and energy management system.
Photovoltaic systems are not the cheapest form of renewable energy on the market – come to that what is? They can be installed as either individual panels, vertically in double glazed units, at an angle on a pitched roof, or in thin film solar roof tiles. As an approximate guide 8m2 of PV will produce approximately 830KWh of power at 1KWp. So to power a house fully from PV would require something in the order of 60 m2 and cost about £36000, saving somewhere in the region of 740kg Carbon per annum. Although more modest PV arrays are normally installed at somewhere around 10% of the annual electricity consumption, which would cost around £6000. One overpowering factor has to be considered when planning the installation of a PV system; they work far more efficiently if sited with a southerly aspect.
Solar Thermal Hot Water Heating:
Similar to PV but employing a water based refrigerant heated by the sun’s rays, which is then pumped to a twin coil hot water cylinder through the top coil – using a pv powered pump – to provide domestic hot water. A south facing solar thermal hot water heating system will provide approximately 80% of all domestic hot water requirements for an all up installation cost of around £3500. As renewable energy sources go this is the most readily accessible by the general public, with carbon savings in the order of 49Kg carbon per annum.
Hydrogen Fuel Cells (FC):
Nolonger in the realm of pure fantasy, FCs use a gas to creat electricity in combination with water by creating a charge from positive and negative electrodes. North Amercia, Germany and Japan are very much in the forefront of both micro FC and major power installation plants, there are micro plants operating succcessfully in the USA but few operate on a sound commercial basis. In the UK the main driving force in FC technology is Rolls Royce who are developing 20MW power plants for providing community based energy solutions. The Major of London invested considerable funds in a Daimler Chrysler designed FC powered bendy bus which was on test in London from 2005 until January 2007.