Pros and cons of REs

To replace conventional energy sources, a renewable energy would have to have to provide:
• Dispatchable power: power needs to be available when the primary customers (the utilities, and their consumers and industrial customers) need it. An energy source should therefore have a high capacity factor. Utility base load plants are designed to achieve power generation over 65% of the hours in a typical year.
• Cost effective power: To be as cost effective as a fossil source, a renewable source must produce power at about $0.10 per KWh. Peak load power can afford somewhat higher costs of around $0.12 - $0.15 per KWh.
• Reliable power: Any source vying to replace coal-based electric power should be as reliable as that technology.

Solar PV is not likely to scale up massively in the near future because of its high cost and variability. Wind power is also unlikely to scale beyond about 10% of our grid electricity needs partly because of its high variability.

Geothermal can provide another 10% and because of built in heat storage it can meet many utility requirements if it can be produced cost effectively. Enhanced geothermal energy is the solution to scalable geothermal power. Enhanced geothermal is simply an extrapolation of naturally occurring hydrothermal systems, using artificial geothermal wells. It operates as follows:
1. Drill a production-injection well into hot rock (the rock in question should have limited permeability).
2. Inject water into the well at a pressure high enough to cause fracturing, until fractures extend a significant distance from the initial well.
3. Drill multiple injection wells around the initial production well, with the intent of overlapping the fracture system
4. Circulate water to capture the heat from the rock, which can then be used to generate power.

Solar thermal concentrates the sun's rays to heat a fluid to generate steam that can then run a regular steam turbine. Its great advantage is that the heat can be stored cheaply either as steam, hot water, molten salt or hot oil and used when the sun's heat is not available. Storing this energy using batteries would be prohibitively expensive and battery technology costs are not declining very rapidly.

Advantages of solar thermal energy compared with other sources of renewable energy
• Low price-variability and supply-availability. CSP bears no transportation, supply or commodity price risk.
• Low land requirements and high siting potential. The total space required to power Europe would be equivalent to about 3% of the land of Morocco. The sun intensity constraint could be reduced by building a high voltage DC power grid for long distance electricity transmission.
• Decreasing cost. Capital and operating costs are both likely to be lower than nuclear. Solar thermal is about 75% cheaper than solar PV.
• Ability to reach capacity factors of 65% and hence supply base power needs.

Source: V. Khosla