One of the problems with so-called "renewable energy sources" is that they are intermittent. Obviously the sun only shines in the daytime, and the wind doesn't always blow. The result is that much more intstalled capacity is required. In Europe experience is showing that the much more excess installed capacity is required than was originally assumed. Here is some discussion on this from the blog "Greenie Watch."

BASIC IDEAS ABOUT ELECTRIC POWER SUPPLY: CAN WE SURVIVE ON WIND, SOLAR AND MARINE POWER?

An email from Prof. Cliff Ollier [cliffol@cyllene.uwa.edu.au], School of Earth and Geographical Sciences, University of Western Australia

ELECTRIC POWER is used for different purposes at different times, and we have to distinguish between average load and peak demand. Sometimes the peak demand occurs in winter evenings (heating loads) and sometimes in the middle of summer (air-conditioning loads).

The Capacity factor is used to compare the relative merits of different types of power supply. It is the ratio of the average load to the peak demand.

CONVENTIONAL POWER GENERATION

There are basically three types of generating plant:

a) Base load that operates ~90% of the time generating efficient low-cost electricity. Coal and nuclear plants are prime examples.

b) Mid-range plants that are often shut down in the early hours of morning and generate maximum power during the day and during peak demand periods. Hydropower and gas-fired stations, are examples.

c) Peak load stations that operate for between 1% and 20% of the time during peak demand periods. Gas turbines, hydropower stations and pumped storage hydropower are examples.

All these plants can be relied upon to operate when needed - unless they break down or fail to start, which can also happen in alternative energy production. Most conventional power systems have a capacity factor of between 50% and 70%.

RENEWABLE ENERGY POWER GENERATION

The currently popular renewable energy technologies add to the problems of operating a power system because they are unpredictable and their output changes rapidly.

Wind power. A change in the output of 50% in a few minutes is not unusual. Attempts to predict the output of wind farms more than an hour ahead have not been successful. Capacity factors vary from 18% to 37%. Wind power costs about US$2200 per KW. This transfers to a cost of 8 -10c /kWh.

Solar power. The output varies predictably every day and unpredictably every time a cloud passes over the sun. A cloud can drop the output by as much as 60%. The capacity factor is around 20%. The capital cost is in excess of $5000 per KW. This transfers to a cost of around 40 c/kWh.

Marine power. Suggestions for using marine power come in many forms but all are very expensive to build, more or less unpredictable, and in most reliability is likely to be low. Operation and maintenance costs are unknown but likely to be very high. The much-touted Pelamis wave power generator project off the coast of Portugal has been abandoned because of financing and technical problems. In the UK the Severn Barrier project to use tidal forces is on hold. It would be the most expensive alternative energy project, and makes a barrier to shipping and fish migration. The tides are reliable, but occur at a different time every day.

None of these renewable energy technologies would exist without grants and massive subsidies.

FURTHER CONSIDERATIONS

Two further considerations are essential in power supply, though they are almost always ignored.

Frequency keeping . Power systems have a need for frequency keeping because the amount of electricity generated must always match the demand exactly. Generating plant must be available that can increase or decrease its output very rapidly to avoid system collapse. This is necessary if there is a sudden large change in load -- the beginning and end of a popular TV programme is a classic example. Frequency keeping stations are designed to cope with these fluctuations.

Energy storage. Renewable energy like wind or solar is not produced when needed, so storage is needed, and this is expensive. All the promoters of renewable energy ignore the need for storage.

What is needed is a large-scale, efficient, low-cost technology that can store huge amounts of electrical energy for weeks or months. No suitable technology exists or has even been contemplated. Hydro-pumped storage is the best we have. It is expensive - at least $1500 /kW – and requires two very large storage lakes not far from each other and with one lake something like 700 m higher than the other. The losses are 25%. The cost, the losses, and the difficulty of finding a suitable site are insuperable barriers to large-scale adoption of hydro-pumped storage.

So people who tell us that it is possible to run modern power systems from wind power, solar power and marine energy are not telling the truth.