Tuesday 20 December 2011

Slippery When Watt

Don’t know your megawatts from your kilowatts peak?  You’re not alone.  Look no further for help.

What’s a Watt?
Some watts are more equal than others
The watt is the unit of power – the rate at which energy is converted from one form, for example coal, to another, for example electricity in a power station. 

According to its rating plate, my kettle at home converts electrical energy to heat energy to keep us going in tasty hot beverages at the rate of 2.2 kilowatts, or 2,200 watts.

1 kilowatt (kW) = 1,000 watts.

At the other end of the scale, my nearest power station at Little Barford, is busy keeping the nation’s television screens alight by burning natural gas to make electricity at the rate of 680 megawatts, or 680,000,000 watts – enough to run 300,000 of those kettles at the same time.

1 megawatt (MW) = 1,000,000 watts.
So if you read that 650 megawatts of photovoltaic solar panels were installed under the Feed in Tariff during 2011, you can compare this to a power station in terms of energy output.  Right?  Wrong!

Give me a ‘p’
A fossil fuel fired power station could in theory run day and night every day of the year at or near full rated output.  The decision whether to or not is typically economic, or influenced by down time for maintenance.

In contrast, a photovoltaic solar panel makes electricity during the hours of daylight, and the output will vary with the position of the sun and cloud cover.  Likewise a wind turbine will produce its maximum (rated) power for a limited range of wind speed.  As the wind speed increases or decreases from there, the power output falls.

The difference between a conventional power station and a solar or wind farm is one small letter that is often dropped from news articles and statistics about this kind of renewable energy.  The letter ‘p’.
A solar panel power output is rated in watts-peak (Wp).  The electricity output of the panel is measured under very specific conditions of light and temperature – equivalent to a clear summer’s day at noon with the panel at 25C.  It is a useful measure to allow comparison of one product from another – it absolutely does not reflect any kind of average power output.

The scale of a solar installation such as a solar farm is the sum of all of the watts-peak of the solar panels used.  Again, a useful comparison between solar farms, but not between solar farms and conventional power stations.

Capacity Factors

The ratio of the energy output compared to that which would be expected if operating at full “nameplate” capacity the whole time is termed the Capacity Factor.
For the entire generating stock of power stations, it is necessary to be able to meet the peak electricity demands of the nation.  This means that at all other times, some of that generating capacity stands idle, resulting in an overall capacity factor between 50% and 60%. 

(Figures from Digest of UK Energy Statistics -DUKES table 5.10)
The energy output from a solar panel installation will depend upon its location, orientation and shading.  For well-situated PV installations in the UK, a rough guide is to expect around 850 kilowatt-hours (kWh) of energy for every installed kilowatt-peak of PV panels.  This corresponds to a capacity factor of around 10%.
Wind turbines at sea benefit from higher average wind speeds.  DUKES table 7.1 allows us to add the average capacity factor for wind energy.

Average Capacity Factor
Gas fired power station
Wind Turbine (offshore)
Wind Turbine (onshore)
Solar PV (UK, S Facing)

(Calculations can be viewed on this spreadsheet)

 All Watts are Equal...
Based on the above, 1 MW of gas fired power station produces the same annual energy output as 2MWp of offshore wind, 2.7 MWp of onshore wind or  6 MWp of solar photovoltaic panels.

The 650 MWp or so of solar photovoltaic panels installed in the UK during 2011 causing such panic in government circles is equivalent to a sixth of Little Barford power station.