Solar installations in the UK.


It is no doubt safest to assume that any correlation between an increase in solar generation in the UK and declining consumption of electrical energy from the grid is no more than that; nevertheless, a correlation is suggestive, and it could be instructive to pursue it further. To do this we will need to know more about solar installations in the UK.

Ofgem, the Office of Gas and Electricity Markets for the UK, publishes data on the total FiT PV installation capacity for each year.
Capacity in kW, as of 30 September 2017 is given as follows (here rounded to the nearest thousand).
Community                                      287,000
Domestic                                          2,743,000
Non Domestic (Commercial)      2,464,000
Non Domestic (Industrial)           367,000

In the above table, the units are capacity in kW. What does this mean? The National Renewable Energy Laboratory (NREL), a laboratory of the U.S. Department of Energy, defines the capacity (of a power plant in general) as the maximum output of a power plant. For PV systems, we would expect maximum output to occur under ideal sunlight conditions. PV systems generate DC, which normally has to be converted to AC to supply appliances. Some power is lost in conversion, and this means that capacity can be measured in two ways: a DC rating before conversion, and a (lower) AC rating after conversion.  I will assume here, as seems likely, that the Ofgem figures for capacity refer to the power available after conversion (the AC rating).

Knowing how much power a solar installation can produce in bright sunshine is only a starting point. Summer sun is brighter than winter sun, and summer days, away from the equator, are longer than winter days. Given a geographical location, however, the average energy delivered by a solar PV installation over an average year can be usefully predicted. The conversion term is called the capacity factor of the system (in a given location).

NREL defines the capacity factor as a measure of how much energy is produced by a plant compared with its maximum output. It is measured as a percentage, generally by dividing the total energy produced during some period of time by the amount of energy the plant would have produced if it ran at full output during that time. https://www.nrel.gov/docs/fy13osti/57582.pdf

An example: suppose that a PV panel produces 1 kW of AC power in optimum sunlight in a certain location. It therefore produces 1 kWh of energy in one hour, and if the sun shone at the same brightness every hour for a year, its annual output would be 8766 kWh, since there are 8766 hours in a year. However, on average the day length is only about twelve hours, the sun is only near its highest for a short time, and clouds come and go. We should therefore expect much less than 8766 kWh. If the installation gave, say, 1000 kWh over the year, its capacity factor would be 1000/8766 X100%, i.e., 11.4%, a figure not far from that of typical installations in the UK.

In an article posted on June 20, 2014 entitled Estimating Global Solar PV Load Factors, Roger Andrews points out some of the difficulties associated with interpreting solar PV data. http://euanmearns.com/estimating-global-solar-pv-load-factors/

One difficulty is that solar installation was proceeding so fast at the time of writing that statistics were difficult to maintain, another that the output from many installations was unmonitored, and a third that fixed position solar arrays (the majority) were not always distinguished from those that tracked the sun’s movement, and so had higher capacity factors. Nevertheless, he surveyed a global selection of monitored solar PV installations to arrive at more reliable figures for variation of capacity factor with latitude. Equatorial arrays produced factors of around 14%, restricted by cloud cover; latitudes between 15 and 35 typically yielded 16 or 17%, declining to around 12% at latitude 50 degrees north and 10% at latitude 60 degrees north, these last figures taking in mainland Britain. There are few data for the high southern latitudes.

In another report, the UK capacity factors (also called load factors) have been estimated at between 10 and 11% in the years 2012 to 2016.

Comments

Popular posts from this blog

Energy maps and calculators

Footprints and Offsets

Climate fiction and climate action