Renewable DEThe various renewable DE technologies include:
- Solar photovoltaic panels
- Roof-top/local wind turbines
- Small-scale local hydro power
- Geothermal energy
- Renewable energy powered fuel cells
- Thermal based technologies:
- Plug-in electric hybrid vehicles
Decentralized versus centralized renewables.Centralized renewables are a move in the right direction but decentralized renewables are the destination. There are several examples of applications where renewable technology cannot be considered DE such “PV farms” or off shore wind farms.
Comparison of decentralized and centralized renewable solar technologies
This small PV panel in the USA shows a typical example of renewable DE because all the power generated from the panel is used locally to pump water for a livestock tank.
Comparison of decentralized and centralized renewable wind technologies
Biomass is a special example of renewable DE because it overlaps also with the other category of DE: cogeneration. In other words biomass (including wood or food scraps, sewage gas, garbage and organic and industrial waste) can be burned for power only (in which case it should not be considered DE) or burned in a cogeneration application creating both useful power and heat. Ideally biomass, like fossil fuels, should always be used in a cogeneration application.
High efficiency cogeneration can include fossil fuelled technology or some carbon neutral bio-fuels (gas, liquid or solid). The various DE technologies that can be used in high efficiency cogeneration include:
Cogeneration (otherwise known as combined heat and power) can be defined as the simultaneous production of electricity and useful thermal energy. Large central generators typically convert about a third of the energy in every unit of fuel into energy that is used with the remaining two thirds going up the stack as waste heat. Cogeneration can double this efficiency by making use of the heat produced whenever fuel is burned to produce electricity. By its nature cogeneration must be employed where electricity is required because if waste heat is to be put to use it must be used locally- it is very inefficient to transport waste heat over distances. Waste heat can be used for such purposes as space or water heating, industrial processes that require high temperatures or cooling and refrigeration (in conjunction with an absorption chilller).
The figure below shows how cogeneration improves efficiency and environmental performance by putting normally wasted heat to use. With CHP only 120 units of the initial fuel is wasted to meet energy demand. This is in contrast to separate production of thermal and electrical energy where almost three times as much energy is wasted to meet the same demand. The same principle applies no matter the scale.
Cogeneration applications can be implemented in a wide range of scales from very large installations of several hundred MW- comparable in scale to large central plants, all the way down to the size of a washing machine for use in individual homes. What all these applications have in common is not the size or technology but the fact that they make use of heat that would otherwise have been wasted.
The below photographs illustrate the wide range of scales at which high efficiency cogeneration can offer benefits.
Cogeneration applications can employ any number of fuels, from fossil fuels such as natural gas and coal or coal gas to methane from waste water plants or wood chips. Any combustible fuel can be used – some technologies lend themselves better or worse to particular fuels. In the case of fuel cells fuels are not burned but rather reformed to produce energy via a chemical reaction.
There is great potential for utilizing waste streams in industry to generate clean electricity without additional fuel consumption or emissions. The industrial sector accounts for about a third of total global primary energy consumption and much of this energy is unnecessarly wasted.
Industrial energy recycling can be used to turn hot exhaust gases, low-grade fuels (some of which are typically flared) and steam/gas pressure differentials into valuable energy and great savings for the factory owners. Hot exhaust from any process can generate steam that drives turbine generators. Coke ovens, glass furnaces, silicon production, refineries, natural gas pipeline compressors, petrochemical processes, and many processes in the metals industry vent exhaust that can be profitably recycled. The energy potential in pressurized gases also can be profitably turned into electrical energy. Examples of pressurized gas include steam, process exhaust, and compressed natural gas in pipelines. The table below provides an estimation of the amount of electricity that could be generated in the United States if some of the industrial energy recycling potential was realized. Similar potential is realizable in any country where industry is an important part of the economy.
The pictures below show two examples of projects making use of energy that otherwise would have been wasted.