The overwhelming advantage of solar is that the energy resource is well in excess of any other renewable or fossil fuel and the source energy is close to limitless.
The practical limitation is that the cost of capturing the energy and transporting it in a useful form (as electricity or perhaps hydrogen) is many times higher than that of already available energy resources.
Terrestrial solar suffers from an inherently lower capacity factor than a good wind province (17-30 solar: 20-35 wind) and is also affected by macro-location issues due to clouds; hours in the day; horizon and shading issues. Atmospheric absorption (air mass) falls with altitude but rises with the angle of the sun below the zenith (directly overhead). Outside the tropics air mass absorption is strongly affected by latitude, particularly when the sun is low in the sky early and late in the day.
Thus the desert areas of Australia, India and Africa and South America (particularly at higher altitudes) are optimum solar provinces. A good deal of the US has excellent solar incidence.
Solar suffers similar intermittency issues to wind. In many urban (non-desert) areas cloud cover and rain seriously reduces the capacity factor and of course the solar capacity factor is seasonal, particularly outside the tropics.
Except for air conditioning, solar energy is not available when it is most needed by consumers (in winter and at night) and too intermittent, without storage, for use by industry. Thus an additional storage cost needs to be factored in and for solar, often the cost of additional long distance electricity transmission from desert or high altitude provinces.
Photovoltaic (PV) solar is already widely used ‘off grid’ for small and emergency power supply and feed-in applications. The main advantage of PV solar is, relatively maintenance free and reliable, direct electricity generation; suitable for charging batteries.
Extracting about four times the energy from a given area of PV solar panel without increasing the cost; reducing the cost fourfold; or combining solar at low cost with another purpose (like incorporating solar collection into windows, roofing materials and/or wall panels) could make PV solar price competitive with current generation wind technology.
In Australia marginal solar will become progressively more attractive to large scale generators if/when the value of a REC (or equivalent ETS) rises above $200.
It seems evident from the literature that researchers are more confident that the cost of PV Solar (based on the cost of electronic technology) is more amenable to future cost reductions than either wind or thermal solar.
For example: modern cadmium telluride (CdTe) thin film photovoltaic (PV) solar cells presently cost about US$110/m2 and can produce around 100 to 150 watts per m2. Higher efficiencies have been achieved in the laboratory (up to 20%). This would increase the energy collected over each square metre. Production improvements and higher volumes might lower the pre installation panel cost to say $0.50 per square metre. To this needs to be added the cost of installation, support-structures cabling and so on; but intelligent building design might incorporate these into roofs or walls. In a ‘real-world’ environment, with a good solar incidence (low latitude/high altitude) and capacity factor (low shading /low cloud environment), it is hoped that a cost per kWh of 8 US cents can be achieved within a decade. At this point PV Solar may become price competitive with current large scale (optimised) wind.
The main competing PV technology is the more mature crystalline silicone. In general this is presently less amenable to further cost reduction but mono-crystalline silicon designs may be able to achieve equal or higher efficiency.
In the large scale renewable sector wind has presently substantially outstripped solar due to lower capital cost per kWh delivered but anticipating higher value RECs some wind projects propose a PV solar co-generation installation. This may serve to better smooth fluctuations in either supply and better utilise transmission infrastructure.
Apart from PV solar there are numerous thermal solar designs. These typically use mirrors or occasionally lenses to focus the sun’s energy onto an energy absorbing element. This may be located on a tower or directly in front of a parabolic mirror. The absorbing medium (high temperature liquid and/or gas) then transfers heat to an engine that drives a generator or performs an energy absorbing chemical separation.
There are fundamental thermodynamic laws that constrain the efficiency of these processes and require very large collection areas in proportion to the solar incidence (insolation). These in turn tend to result in very high capital costs. As with PV solar development work is directed towards improving energy capture per square metre of collector (efficiency) and reducing the cost per square meter of collector. At this stage thermal solar is relatively mature with over 100 years of development but no breakthrough has yet overcome these cost hurdles.
For example, the first solar power station in Australia was commissioned in 1979 at White Cliffs NSW. White Cliffs is in the far North West of the State, adjacent to SA and Qld and was chosen because it has the highest insolation in the State. It consisted of fourteen three-metre parabolic dishes focussed on a collector, where water was boiled to drive a steam engine, delivering up to 25kWe and complimenting the town’s diesel generator. The town was connected to the grid in 1996. At this point the station was converted to experimental photovoltaic (used as grid feed-in). It ceased operation in 2004.
The largest commercial solar power station in Europe is the Andasol parabolic trough solar thermal power plant in Granada, Spain. Andasol 2 went online in March 2009. Andasol 3 is currently under construction. Because of the high altitude (1,100 m) and the desert climate, the site has exceptionally high annual direct insolation of 2,200 kWh/m² (7,920 MJ/m²) per year. Each Andasol plant has a gross electricity output of 50 megawatts (MWe), producing around 180,000 MWh per year. Each collector has a surface of 51 hectares; occupying about 200 ha of land.
Source: Andasol Publicity Shot
Solar plant is considerably more physically compact than wind; compare Andasol with the Capital Wind Farm generating 450,000 MWh per year over a total site of 35 square km (but less equipment overall).
Each unit has a molten salt thermal storage system which absorbs surplus heat produced at midday. A full thermal reservoir (in summer) can continue to run the turbine for about 7.5 hours at full-load after sunset. Energy storage improves the capacity factor of the Andasol facility to a claimed 41%, with a considerably less fluctuating power curve than non-storage configurations. Each Andasol plant cost approximately AUS$500M ($10,000/kW) and the commercial viability is said to depend on a huge subsidy; equivalent to between $400 and $600/MWh (in Australia equivalent to a REC + ETS price of $400 to $500).
By comparison a 2,000 kW wind turbine costs around one seventh of this per rated kW. The Capital Wind Farm cost approximately $210 million ($3.1 million per turbine installed; $1,493/kW capacity). After adjusting for its improved capacity factor, due to thermal storage (but not taking into account differences in maintenance or capital servicing), Andasol solar power is around six times the delivered price of electricity from the Capital Wind Farm.
In Australia an advanced solar thermal pilot plant under construction at Newcastle is based on 450 collecting mirrors (4000 m2) together with a 30 m high tower and associated heat engine (turbine and generator). The projected output is 200 kW at a (pilot) cost of $5 million ($25,000/kW – two and a half times that of Andasol).
In NSW similar cloud free solar provinces to Andasol are many times more distant from the grid than proven wind resources. White Cliffs is the only town in NSW with similar insolation; but there are less remote areas of Australia (in Qld, SA, and WA) that are similar or better solar provinces. These may well become economic before any in NSW.
Together these factors that suggest it will be a considerable time before large scale thermal solar becomes competitive with wind in NSW; even with a REC price rising (in combination with an ETS) to provide a subsidy comparable to that supporting Andasol.