Latest version – feel free to comment or edit – strangely, has a large number of refrences from the claverton site.
An intermittent energy source is a source of electric power generation that may be uncontrollably variable or more intermittent than conventional power sources, and therefore non-dispatchable, and is usually used to refer to sources of renewable energy such as wind and solar generated electricity.
At present, the penetration of intermittent renewables in most power grids is low, but wind for example provides nearly 20% of the electricity generated in Denmark (where plans are underway to increase this substantially)  and 7% in Germany. The use of small amounts of intermittent power has little effect on grid operations. Using larger amounts of intermittent power may require upgrades or even a redesign of the grid infrastructure. However, it should be borne in mind that large amounts of large inflexible plant, such as nuclear or supercritical coal or coal plant equipped with CCS will also require significant grid upgrades to deal with the inflexibility.
Technological solutions to deal with intermittency already exist and studies by academics and grid operators indicate that the cost of compensating for intermittency is expected to be low even at levels of penetration substantially higher than those prevailing today.  Large, distributed power grids are better able to deal with high levels of penetration than small, isolated grids. For a hypothetical European-wide power grid, analysis has shown that penetration levels as high as 70% are viable, and that the cost of the extra transmission lines would be only around 10% of the turbine cost, yielding power at around present day prices  Smaller grids may be less tolerant to high levels of penetration.
Matching power demand to supply is not a problem specific to intermittent power sources. Existing power grids already contain elements of uncertainty including sudden and large changes in demand and unforeseen power plant failures. Though power grids are already designed to have some capacity in excess of projected peak demand to deal with these problems, significant upgrades may be required to accommodate large amounts of intermittent power.But again, large amounts of large inflexible plant will also require grid upgrades and increased interconnection – ie the French nuclear programme necessitated its 2 GW HV link ot UK, and increased imports and exports to neighbouring countries. The International Energy Agency (IEA) states: “In the case of wind power, operational reserve is the additional generating reserve needed to ensure that differences between forecast and actual volumes of generation and demand can be met. Again, it has to be noted that already significant amounts of this reserve are operating on the grid due to the general safety and quality demands of the grid. Wind imposes additional demands only inasmuch as it increases variability and unpredictability. However, these factors are nothing completely new to system operators. By adding another variable, wind power changes the degree of uncertainty, but not the kind…”