District Energy networks derive significant benefit from economies of scale, drawing together multiple diverse loads to provide an energy base-load which high-efficiency, low-carbon plant can operate against. The potential for carbon and cost savings is realised by the choice of plant which supplies the energy (heat, cooling or electricity) to consumers, via the district energy networks. The networks themselves are “fuel agnostic”. Therefore, as new technologies become viable (such as fuel cells or the latest generation of biomass CHP) these can effectively be “plugged in” to the existing network and serve the aggregated loads of the district energy scheme. This approach is far more effective than relying on action by individual consumers, which could not provide the same level of carbon and cost effectiveness, especially in the same timescale.
Some typical technologies used with district energy schemes are listed below:
Gas Fired Combined Heat and Power (CHP)
CHP is the generation of on-site electricity whilst simultaneously utilising the waste heat from the process for useful purposes, such as heating water. Heat is always produced during power generation but is usually not recovered for useful purposes, giving an overall efficiency for the conversion of fuel into useful energy of just 30% - 40%. By recovering the heat produced in the process, the overall efficiency of energy conversion can be increased up to 80% - 85%.
A CHP unit typically consists of either a reciprocating engine or a turbine, running on either natural gas or diesel. The combustion of the fuel within the engine creates motive power, which is converted into electricity via the alternator. Exhaust gases typically leave the engine at approximately 500°C. A heat exchanger placed in the flow of exhaust gases transfers this heat to a working fluid, typically water, which is heated to a high temperature or raised to steam. Low temperature hot water (LTHW) can also be recovered from the engine jacket cooling circuits and the aftercooler.
Biomass is fuel derived from living or recently living material, such as wood, energy crops and certain plant oils. Combustion of biomass fuel to produce heat releases carbon dioxide to atmosphere but these emissions are offset by the carbon dioxide originally absorbed during the growth of the biomass. Biomass is not entirely carbon neutral however as further greenhouse gas emissions can be associated with the harvesting, transport and processing of the fuel. Land use change for biomass cultivation (for example, forest clearing for palm oil plantations) can also have significant environmental impact.
With all of these considerations correctly taken into account, combustion of carefully chosen biomass fuels has the potential to make significant savings in carbon emissions, when compared to combustion of fossil fuels.
So called “second generation” biomass fuels are often liquids or gases, derived from processing biomass feedstocks in some way, such as anaerobic digestion or pyrolysis. These processes can often use organic waste as a feedstock, and therefore do not necessarily result in the same impact on land use change or competition with food production as some first generation biomass fuels.
As with gas fired CHP, biomass CHP utilises fuel more efficiently. Typically, the biomasss fuel is combusted to raise steam or heat organic oil, which then drives a turbine. Heat is recovered from the exhaust gases, maximising the energy extracted from the primary fuel.
This highly efficient use of low carbon fuel provides the potential for very significant emissions reductions. Biomass CHP is typically more expensive than its gas fired counterpart, and can require a significantly larger footprint for a system with equivalent output.
Energy from WasteIncineration of waste releases energy which can be used to generate power and usable heat. Regulations can be relatively onerous, particularly regarding emissions of particulates, heavy metals and nitrous oxides. Depending on the waste stream utilised, different proportions of the waste can be considered as biomass, resulting in carbon emission savings.