Biomass on the front burner
14 November 2006
Niall FirthA new project at Nottingham University's School of the Built Environment aims to create the first small-scale biomass-fired combined heat and power (CHP) system, which is said to be ideal for use on farms and in other large domestic buildings.
The project aims to help prove the viability of small-scale CHP generation, according to Professor Saffa Riffat, one of the project's leaders and an expert in sustainable energy technologies and thermal engineering.
'At the moment there are a number of large-scale 100-200kW CHP generators but there are no domestic-sized ones,' said Riffat. 'This will be the first biomass-powered CHP on a small scale.'
The research is being developed at the school's Institute of Sustainable Energy where researchers from a variety of disciplines are also working on the experimental Stoneguard C60 house (see cover feature).
The first stage of the 18-month micro-CHP project, which began this month, is to look at ways of miniaturising the CHP components to enable smaller-scale biomass devices to be built. The project aims to develop a system that only produces up to 10kW, which is more than enough power for domestic use, according to Riffat.
Most CHP systems on the market are powered using fossil fuels, usually gas and oil, which does not sit well with the institute's sustainable energy ambitions. Also, most existing biomass generators are used only to produce electricity rather than as a combined CHP unit. The research aims to find a way of creating heat and power using a biomass-powered furnace.
The institute plans to develop the steam Rankine turbine cycle, which is already the state of the art in biomass-powered energy production technology.
The Rankine cycle uses a liquid that is pumped at a high pressure into a boiler, where it is heated until it becomes a vapour. This vapour is then expanded through a turbine, which provides electricity. Once it leaves the turbine it is quickly condensed back into a liquid then re-enters the pump so it can be re-pressurised and the cycle starts again.
This form of steam-driven Rankine power generation, although useful in large-scale energy production, is of less use in low-power systems because of its inherent low efficiency in energy generation and high capital costs. To make a system that only generates about 10kW, the efficiency would have to be greatly improved. To also provide heat generation as part of the same system Riffat and his team have modified the traditional cycle by using a refrigerant to power the turbine.
'We will have a biomass- powered furnace with a heat exchanger inside filled with a liquid refrigerant such as CO2 or pentane,' said Riffat. 'As a high-pressure vapour it will expand through a turbine at 500-600Cº and the turbine will produce electricity.' Up to this point, the Rankine Cycle remains unchanged.
However the use of the refrigerant adds a new element to the cycle: the production of heat. 'When the refrigerant is condensed and it moves back from vapour to liquid it gives up heat, which can then be utilised,' said Riffat.
The team has produced a miniature 2kW turbine and preliminary tests have revealed this modified design is more efficient and reliable than previous conventional systems. The next step is to couple one of these small turbines with a biomass boiler. Riffat hinted the research may encompass other innovations, including the elimination of the pumps in the cycle, that would make the system even more efficient in the future.
He said there is a large and growing market for a system that could reuse household waste and other biomass materials to produce small-scale localised power and heat generation, particularly in farms. 'If you are using large-scale CHP you have to get a group of houses together where everybody wants to be involved, which isn't always easy,' he said.
The team aims to commercialise the system soon although it does not yet have any industrial partners. The first prototype could be ready in six months, claimed Riffat.
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