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ALTERNATIVE ENERGIES
Our understanding of three alternative heating methods - Ground Source Heat Pumps, Combined Heat & Power, and Wood Fuelled Boilerplant, is summarised below:-
A heat pump is essentially a refrigeration machine taking heat away from a ‘source’ at one temperature and discharging it to a ‘destination’ at a higher temperature.
The ratio of the total heat discharged divided by the energy (electricity) needed to move it is the co-efficient of performance (COP).
The COP reduces as the temperature difference between source and destination increases.
A Ground Source Heat Pump can take heat from the ground using coils of pipework arranged horizontally in trenches, or vertically in boreholes. The temperature of the ground below about a metre in depth stays relatively stable at about 12ºC.
The temperature of the ‘output’ water from the heat pump has a typical maximum of about 45ºC, and for this reason heat pumps are most commonly used with underfloor heating installations, which can be designed to operate satisfactorily with water at this temperature. The alternative is to use very large radiators, much larger than would be required with a boiler operating at up to 80ºC.
Requirements Sufficient land for horizontal trenches for the ground source coils, or to allow access for a drilling rig to drill one or more boreholes.
Heating system suitable for low operating temperatures (eg underfloor heating)
Suitable electrical supplyGood Points Reliable equipment, low maintenance
The stable source temperature ensures a good COPBad Points High installation cost associated with trenching/ drilling boreholes
2 Combined Heat & Power Equipment
Essentially an engine driven alternator to provide electrical power with the engine cooling water used for heating purposes, Combined Heat & Power (CHP) is an extremely efficient way of producing usable heat and generating electricity simultaneously at the point of use from a single fuel. In very rough terms, a plant delivering ‘x’ kW of electricity will deliver about ‘2x’ kW of heat.
When a plant is operating to meet the electrical loading it is unlikely to be providing the required thermal output and vice-versa. The use of accumulators or thermal stores (large well-insulated tanks of hot water) will help to store heat when not immediately required. The use of batteries is very expensive and not environmentally friendly. The sale of excess power to the electrical grid is possible, but there can be difficulties in reaching a suitable agreement on the price per unit - often a ‘wholesale’ price considerably less than the ‘retail’ cost to purchase units. Accordingly, a CHP plant would normally be sized on meeting part only of the electrical load, and such that the heat output can be made use of.
There are a number of different types of CHP plants available ranging from small reciprocating engine solutions, to large gas turbine and steam turbine derivatives. By generating heat and electricity from a single source, CHP can deliver overall fuel efficiencies well in excess of 75%. When compared with electricity generated from a centralised power station, and the use of heat only boilers, CHP can reduce primary energy needs by up to 30%.
Smaller CHP plants are generally fired with natural gas or lpg, although
the cost of lpg (typically significantly greater than that of natural
gas) should be noted. When fired by natural gas, they can considerably
reduce energy costs and deliver significant reductions in harmful greenhouse
gas emissions such as carbon dioxide (CO2).
Requirements Space for plant
Good Points Reliable equipment, moderate maintenance
Reduces dependency on National Grid
Running cost savings, Environmental benefitsBad Points Installation cost
Limited Plant Life (less than 20 years?)
Small scale plants generally run on natural gas
Lpg at much greater expense often not cost-effective
Exhaust/Flue required
Noise
For larger systems, and particularly where there is a source of wood fuel, this type of boilerplant is worth consideration.
Wood fuel is potentially ‘carbon-neutral’ and is a renewable resource. Carbon is locked up in trees as they grow, and is released when the wood is used as a fuel. When wood fuel replaces fossil fuels such as oil and gas, much less carbon dioxide remains permanently in the atmosphere - it is locked up again by the growth of replacement trees. Currently it is not possible to eliminate totally all use of fossil fuels as some will be required to process and transport wood for fuel, but this is considered acceptable in the context of overall emissions reduction and can be minimised by reducing transport distance through local sourcing of wood fuel. There is a growing network of wood fuel suppliers around the country.
Many of the boilers available are supplied complete with fuel and ash handling systems so as to be fully automatic. On some, a small oil or gas burner is fitted to the front of the boiler and is used to ignite an initial intake of chip, switching off automatically after a set time period, and cutting back in if the boiler temperature indicates that the chip has stopped burning.
The boilers operate at similar temperatures to oil-fired plant. Where there is a source of wood, and staff available to operate the system, a wood-fuelled boilerplant can be very practicable. The boilers are relatively slow to respond to controls - you cannot instantaneously increase or decrease the heat output to exactly follow a load profile, and their heat output can be varied within a limited range only - it is difficult to operate them at very low load levels. For these reasons it is normal to decouple the boiler from the load via one or more accumulators / thermal stores. The accumulators / thermal stores provide heat to the system as required, and the boiler tops up the heat in them as and when required, with a fairly sophisticated control system.
Requirements Space required for plant and fuel storage
NOT ‘fit and forget’ - Regular attendance is required to replenish fuel store.
Best suited to larger installations.Good Points Reliable equipment, moderately low maintenance
Running cost savings, Reasonably carbon-neutralBad Points Moderate / high installation cost when complete with automatic stoking and ash removal - ideally require means for fuel to be tipped into a storage bunker higher than boiler
Control and turndown limitations / relatively slow response
Flue required
NOT ‘fit and forget’ - Regular attendance is required to replenish fuel store.