Thermal mass – possibly brine ponds – under the collector zone means that some operation will continue into the night. For solar power, similarly low levels were reached already at 15% penetration. Industrially, probably the main use is in solar salt production – some 1000 PJ per year in Australia alone (equivalent to two-thirds of the nation's oil use). Hence the energy inputs to manufacture are not insignificant.
While sources such as wind, biomass, and solar are transitioning rapidly from curiosity to viability, they only account for about 26.2% of the electricity generated worldwide Moving water is the most important renewable energy source in Canada, providing 60% of Canada’s electricity generation. Technological advances, aggressive policymaking, and heightened public awareness have created momentum to power the evolution of renewable energy. It is possible that where supply reliability cannot be guaranteed, many electricity users placing a premium on reliability (eg, hospitals) will invest in expensive local generating capacity (typically diesel generators).The third category of intermittent renewable integration cost is grid interconnection.
In some systems there is provision for feeding surplus PV power from domestic systems into the grid as contra to normal supply from it, which enhances the economics. In any case wind needs about 90% back-up, whereas the level of back-up for other forms of power generation which can be called upon on demand is around 25%, simply allowing for maintenance downtime.Modelling done by the UK National Grid Corporation shows the effect of wind's unreliability on the required plant for achieving the 20% UK renewables target:Thus, building 25 GWe of wind capacity, equivalent to almost half of UK peak demand, will only reduce the need for conventional fossil and nuclear plant capacity by 6.7%.
While relatively few areas have significant prevailing winds in this range, many have enough to be harnessed effectively and to give better than a 25% capacity utilisation. In 2017, 121 GWh was supplied from pumped storage according to IRENA.
In such a situation significant levels of dispatchable backup capacity are needed to ensure security of supply.Estimates of the wind/solar reserve requirement suggest that the reserve capacity ratio is expected to increase exponentially in proportion to the reliance of the system on intermittent power generation*. CPV can also be used with heliostat configuration, with a tower among a field of mirrors.In 2011 several Californian plants planned for solar thermal changed plans to solar PV – see mention of Blythe, Imperial Valley and Calico below.China's 200 MWe Golmud solar park was commissioned in 2011 and is claimed to produce 317 GWh/yr (18% capacity factor). At the same time, European non-fossil fuel obligations have led the establishment of major offshore wind forms and the prospect of more.However, much environmental impact can be reduced. A Quantitative Renewable Energy Scenario for 2050. Solar energy's main human application has been in agriculture and forestry, via photosynthesis, and increasingly it is harnessed for heat. The economics of VRE are complex, even before system costs are explored.In practical terms non-hydro renewables are therefore able to supply up to some 15-20% of the capacity of an electricity grid, though they cannot directly be applied as economic substitutes for most coal or nuclear power, however significant they become in particular areas with favourable conditions. Ocean Energy Europe reported that capacity installed in Europe since 2010 reached 11 MWe in 2018, with 3 MWe of that still operational. According to the IEA study the only non-VRE investments should be in additional flexible resources – essentially gas-fired, and hence contributing to COMeanwhile Germany provides a case study in accelerated integration of VRE into a stable system, with both politically- and economically-forced retirement of conventional generating capacity. Also, some 30 GWe of spare capacity will need to be on immediate call continuously to provide a normal margin of reserve and to back up the wind plant's inability to produce power on demand - about two thirds of it being for the latter.Ensuring both secure continuity of supply (reliably meeting peak power demands) and its quality (voltage and frequency control) means that the actual potential for wind and solar input to a system is limited. Gas turbines would also emit greenhouse gases as it would likely be prohibitively expensive to install CCS for sources with such low load factors. An EROI of about 7 is considered break-even economically for developed countries. California's 280 MWe Solana uses 125,000 tonnes of salt, kept at 277°C.SolarReserve’s 110 MWe Crescent Dunes plant at Tonopah in Nevada has a 195-metre power tower and claimed molten salt heat storage to enable 10 hours at full load, and 500 GWh per year output (52% capacity factor). Generation III plants have more scope for load-following, and as fast neutron reactors become more established, their ability in this regard will be an asset.If electricity cannot be stored on a large scale, the next logical step is to look at products of its use which can be stored, and hence where intermittent electricity supply is not a problem.Power generation technologies generally compete with each other both in regulated and deregulated markets to supply electricity through a ‘merit order’ based on availability and marginal cost of production for any given period. So while the ability on any individual PWR reactor to run on a sustained basis at low power decreases markedly as it progresses through the refueling cycle, there is considerable scope for running a fleet of reactors in load-following mode.