By Dave Elliott
Some renewable energy sources are variable, so over a year the actual energy output from wind turbines, etc, will be much less than the theoretical maximum if the energy conversion devices was able to work at 100% efficiency for the full time using its full rated power. The ratio of the actual effective capacity the device offers to its nameplate installed capacity is sometimes called the capacity factor, or more usually load factor.
In the UK onshore wind load factors vary from below 20% to above 40% depending on location. 25% is often taken as an average, but that has been moving up to 30% as the technology improves. Offshore wind is sometimes quoted as 35-40% but DECC uses 45% for new projects. The best for far achieved, at Horns Rev II off Denmark, is 47.7%.
PV solar is quoted in the range 10-20% depending on location e.g. its 19% in Arizona. The UK range maybe 10-12% Hydro is site specific: global average 44% but it can be very much higher or lower depending on location and year e.g. down to 22% in poor rainfall years in the UK. Biomass plants can have quite high load factors- since the fuel is storable. So they don’t need backup/balancing- indeed they can be used for that. For some examples of LF’s for current UK renewable energy projects see: www.ref.org.uk/energy-data
And for the DECC’s views on future systems, see below.
DECC’s Load Factors from the UK 2050 Pathways spread-sheet
Offshore wind 45%
Onshore wind 30%
Tidal stream 36%
PV solar 10%
Biomass – electricity from CHP 90%
Geothermal – electricity from CHP 80%
Fuels from biomass 90%
Solar hot water 50%
Bioenergy, energy from waste 80%
Tidal range 23%
For comparison, the 2011 DUKES quotes UK nuclear load factors as: 69.3% (2006), 59.6% (2007), 49.4% (2008), 65.6% (2009) and 59.4% (2010). That averages out at 60%. But optimistically it uses 80-90% for future projects. The NEI quotes average US nuclear load factors (1971-2009) as 70% but again future projects are assumed to reach much higher figures.
Degradation over time
The anti-wind lobby has recently argued that, in practice, wind load factors will reduce by around 1-2% p..a. as the machines get older, due to mechanical wear are tear and blade damage (e.g. under the impact of bugs, birds and debris) and increased maintenance outages. See the data at: http://windfarmrealities.org/?p=1284
However, it’s hard to separate out variations in wind availability over the years from any operational degradation and technical/operational improvements. Danish offshore load factors from 1980 to 2011 mostly seem to indicate continual improvement: http://energynumbers.info/capacity-factors-at-danish-offshore-wind-farms
All mechanical devices suffer from ware and consequent reduced performance over time, but that is likely to be much more pronounced and significant for high-temperature close-tolerance steam and gas turbines i.e. for conventional power plants. While, there is no question that, being exposed to the elements, wind turbine blades, just like aircraft wings, will need cleaning regularly, there are some clever ideas emerging for that, which avoid the need for them to be shut down for the duration to give access to cleaning crews- with all the associated risks of working at height: Opinno have a self powered mechanical system that crawls along the tower and spreads water which cleans the blade when it passes through the spray: That sort of thing will be vital for offshore devices, where salt encrustation will be the key issue: www.opinno.com/blade-cleaning
PV cell performance can reduce over time (especially with thin film cells) and PV arrays also need regular cleaning to avoid loss of output, otherwise, depending on location, you can get maybe 10-20% or more degradation over a year from bird droppings, leaf mould, road grit, dust and the like. With the rapid expansion of domestic PV, there is a growing trade in cleaning services as an extension of window cleaning. Using detergent has negative eco impacts, but some self-cleaning surfaces have been developed (see http://www.solarguide.co.uk/new-self-cleaning-glass-could-be-used-in-solar-panels and http://www.solarpowerportal.co.uk/product_catalogue/self-cleaning_coating_for_solar_pv_glass_could_increase_efficiency_by_up_to).
In addition electrostatic dust repelling systems are being developed for use in with CPV and CSP in desert areas, based on technology developed for Mars missions: http://www.ecoseed.org/technology/13801-mars-inspired-technology-makes-pv-panels-self-cleaning
The longevity of renewable energy technologies also varies. Wind turbines are expected to last a couple of decades before needing replacement although increasingly projects are re-bladed quickly to take advantage of improved, uprated, designs. PV cells also have lifetimes in the decades, but can perhaps sometimes be beneficially replaced earlier by newer more efficient versions. These system, along with modular wave and tidal stream devices, are very different from large very capital intensive very long lead-time projects like tidal barrages, where, once built, the civil engineering infrastructure will last for hundreds of years, with just the turbine generators being replaced every 50 years or so.
Nuclear plants are somewhat similar- they have very long construction lead times but it’s claimed that once built they can run for 40 years for more, with occasional refits and extensions, although that often results in down rating. In addition big inflexible systems like this face the problem that over the long periods that they can be run, rival technologies may emerge which will make them economically and technologically obsolescent. In addition there are large and often unknown back end cost- for waste disposal and final decommissioning.
All of the factors mentioned above are reflected, to varying degrees, in the economics of the various technologies. At present on-land wind is usually seen as the most viable economically of the new renewables, established hydro and biogas waste plants aside, and as being competitive with new nuclear, with PV seen as likely to come up fast behind, followed by offshore wind and then hopefully tidal stream and perhaps wave . Opponents of wind /PV etc say the cost estimates do not take into account the grid balancing/backup intermittency costs, while opponents of nuclear say they don’t take back end and insurance cost fully on board- or the cost of providing extra reserve capacity to cope with the risk of nuclear plant outages. For the moment, in reality, the difference may be too close to call, in which case decisions should really be based on wider strategic issues – such as long term energy security and fuel costs and availability. On that basis, although as I’ve indicted there are some practical problems, overall renewables seem to win outright- for most, the fuel is free and will be available indefinitely into the future , while fossil and nuclear fuels will inevitably become more constrained and expensive.