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Renewable energy sources – when they have become commercial – have their part to play. Engineers and scientists have made remarkable progress in the development of concepts, materials, systems and technologies. But the exaggerations and distortions regarding the possibilities follow a political agenda. Fundamentals and common sense are discarded in the fervour – almost religious – of “environmentalism” and “global warming” and subsidy scams. The realities of what renewables can offer is far from the rosy perceptions that prevail.
It is worth just reminding ourselves of the fundamental constraints which apply:
Generating Capacity: Wind and solar capacity require full back-up capacity but hydro power does not.
- Wind power is intermittent and cannot be predicted. Therefore generating capacity needs cannot rely on wind power capacity and 100% back-up in the form of alternate capacity is always needed. Since electrical power cannot be stored, wind power cannot follow load needs. Any variation in wind power produced must be compensated for by changing the power generated by some other plant in order to follow load. Wind power cannot be despatched.
- Solar power (thermal or photovoltaic) is intermittent not only between day and night and between winter and summer but also during the hours of sunshine due to clouds, rain and dust storms. Some little storage of thermal energy (molten salts for example) is possible but storage of electrical power in batteries or the like is not
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feasible.
Solar plant capacity must also be backed up by alternate generating capacity and since this falls to zero every night, the back up required is also around 100% (with some variation due to the particular night time load profile). Because thermal storage can be available some load changing during daylight hours is feasible.
- Hybrid solar thermal – fossil fuel plants can ensure continuous operation and eliminate the back up capacity.
- The lifetime of components in a solar thermal plant is drastically affected by the enforced cycling caused by daily starts and stops. (Material fatigue and creep considerations are determined by thermal cycling).
- Hydro power plants are dependent upon seasonal water levels in reservoirs for large plant or on variations of water flow in smaller run-of-the-river plants. Large plants are nearly always used for base load power (when in-season) and can also be used for power storage of surplus power from other plants if equipped with a pumped-storage facility. Hydro power plants are always included within the generating capacity base and require no back up capacity. However a grid’s load changing needs (to follow load) must usually be provided for by other types of plant (gas or coal).
Availability and capacity factor:
- Wind power is available only when the wind blows above a minimum value (around 4 m/s) and below a maximum value (around 25 m/s). It cannot operate in gusting conditions. For safety considerations ice formation on turbine blades must be avoided and this gives a minimum ambient temperature for operation as well. Though wind turbine machinery may be available to operate for over 90% of time, the wind or weather conditions are the limiting factor and a wind turbine – dependent on siting – can usually generate power for not more than about 40 -50% of a year. But it is not possible to predict when it will be in operation and at what load. The resultant capacity factor for a wind turbine is around 20% (i.e. a wind power plant only generates about 20% of its rated capacity on an annual basis).
- Solar thermal plants without storage can operate for about an annual average of 8 -9 hours per day. With thermal storage they can operate for about 14 or 15 hours per day and where the solar field is used to augment a fossil fuel plant continuous operation is possible. Without storage, a solar thermal plant has a capacity factor of around 20% which can be increased with thermal storage to about 40%. Currently the cost of thermal storage adds about 75% to the cost of a solar thermal plant.
- Solar photovoltaic plants cannot use any form of energy storage and therefore have a capacity factor of around 20%
- Large hydro plants running at base-load have capacity factors well above 80% (in-season).
- Small run-of-the-river hydro plants can have capacity factors ranging from 30% in seasonal flows and over 80% in perennial flows.
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