This cycle is the simple gas turbine cycle, called the Brayton cycle. However, additional equipment and techniques can be used to increase the efficiency of the cycle. These modifications include: regeneration, intercooling and reheating.
Typical microturbines have efficiencies of 25%–35%. When used in a CHP system, they can achieve efficiencies of greater than 80%. Microturbines have additional design considerations to take into account, however. As a general rule, microturbines have to be designed with the premise that once installed they will receive very little maintenance, except possibly an annual service. But they will need to be reliable despite this. In addition, microturbines need to be small, limiting opportunities for peripheral equipment, and they need to be quiet, limiting opportunities for additional rotating machinery. These factors tend to limit the opportunities to increase turbine efficiencies. There is a limit, for example, on the closeness of blade tips and the casing because of the potential variation in operating regimes. Methods of reducing blade fouling are limited to annual services.
In general, efficiency is influenced by:
- energy used by the air compressor – if less energy is used to compress the air, more energy is available at the output shaft
- temperature of the gas leaving the combustors and entering the turbine – the higher the temperature, the greater the efficiency
- temperature of the exhaust gas from the turbine – the lower the temperature, the greater the efficiency
- mass flow through the gas turbine – in general, higher mass flows result in higher efficiencies
- pressure drop across inlet air filters – increased pressure loss decreases efficiency
- pressure drop across exhaust gas silencers, ducts and stack – increased pressure loss decreases efficiency.
There has been considerable work done to improve the efficiency of gas turbines, mainly on increasing turbine entrygas temperatures and increasing the efficiency and capability of the compressor. Various methods have been used to improve efficiency in these areas. These include:
for New Zealand dairy farms
Dairy farms could form an ideal application for cogeneration plants fuelled by biogas produced on-site by the anaerobic digestion of manure. Such a solution would cut farmers’ electricity bills and help to solve a waste disposal problem, writes Ian Bywater.
Dairy farming in New Zealand has expanded rapidly, with dairy cattle numbers growing by 23% between 1999 and 2002, and more growth expected. A less welcome result is that a lot more manure has to disposed of at milking time.
Between 6% and 12% of the manure produced by a New Zealand dairy herd each day accumulates in the dairy shed area. This quantity is much higher in countries such as the UK, where the animals are not out grazing everyday as is the case in New Zealand. Therefore if the New Zealand system is shown to be viable it will be of even more use in countries where animals are housed for part of the year. This is because more manure is then available for collection.
From the 1970s, New Zealand farmers have been encouraged to dispose of effluent from dairy sheds in a two-pond treatment system. The ponds receive the waste water effluent daily after milking. They contain manure, spilt milk, water from udder and machine washing, chemicals used during milking, mud and grit. About 30% of New Zealand dairy farms still use this system, but it does not always dispose of the raw effluent or destroy pathogens to today’s environmental standards.
In the 1990s, following the passing of New Zealand’s Resource Management Act, the Ministry of Agriculture and Fisheries stopped promoting effluent ponds. It now recommends disposing of diluted effluent by spray irrigation of open pasture, sometimes after storage in an effluent pond if conditions are not suitable for daily dispersal. Problems with this solution include the contamination of groundwater, leaching of nutrients from the soil, delay in grazing, and unpleasant odours. In some situations, spray irrigation is impractical or constrained by soil type. This means that ponds must be used, preferably those with an advanced design and function.
Dairy farming places a considerable load on the electricity grid once or twice daily at peak times
Dairy farming also requires large quantities of water each day to keep the dairy shed and environs clean, and cold water is needed to pre-cool the milk in an amount estimated at 50 litres per cow per day. Groundwater is first used to pre-cool the milk and then held for the shed hosing operations.