Radiation Global radiation Direct Normal Radiation Diffuse Radiation Absorption by Clouds and Pollutants
Figure 1.22 Direct radiation arrives on a clear sunny day; diffuse radiation arrives filtered and reflected by clouds, pollution and water vapour.
Figure 1.23 Measuring
solar radiation with an electronic solar insolation meter.
Direct and diffuse radiation
Another factor influencing the availability of solar energy is how much direct, as opposed to diffuse, sunlight is available at a location. Direct sunlight (sometimes called beam radiation), when the sun is not obscured by clouds, is the most useful form of solar radiation, especially for the purposes of heating (infrared radiation). But daylight is also scattered by particles in the atmosphere. This is called ‘diffuse radiation’ and it too can be useful for lighting and a reduced amount of PV power. On average, over a year, in northern Europe, half the total solar radiation is direct and half diffuse (Figure 1.22). The term ‘insolation’ can be used to describe direct, diffuse or total radiation on any orientation of the surface.
Estimating the amount of energy available
This introductory book is too general to give sufficient information to enable calculations to be made for specific projects at specific locations. There are companion books available in this series that provide this data for off-grid and grid-connected photovoltaics and solar water heating on a small/medium scale, and in Chapter 10 Resources are listed websites where detailed insolation figures can be found (see Figure 1.23). A wealth of data is available to enable calculations
is fed into functions that take into account variables, such as the azimuth (the direction of the sun, expressed as the angular distance from the north or south point of the horizon to the point at which a vertical circle passing through the object intersects the horizon), latitude, seasonal variation, climate, orientation and so on.
The Resources section also contains references to software, available both freely and at a price, for making calculations based on specifications for systems, buildings, materials and insolation data.
Figure 1.24 A parabolic solar thermal dish in Arizona, US, focusing direct infrared heat at
around 1000°C (1832°F) onto a Stirling engine, to generate electricity.
In Chapter 4, in the section on concentrating solar power, the world’s first solar power station is described. I tell the story of its success and readers may be surprised by the date of its operation – 1913. I reveal how it was unfortunately abandoned as the First World War plunged the globe into chaos. This war was partially about oil. The so-called ‘Great Powers’ fought over access to the newly discovered oilfields near Kuwait. Its causes and outcome have much in common with many conflicts that have happened since then, not least the much more recent first and second Iraq Wars. Nowadays, the case for renewable energy is being made in the name of energy security. But only a complete abandonment of oil and all other fossil fuels as a source of energy, and a return to decentralized energy, of a nature that is appropriate for every given location, will give us true global energy security and an end to wars based on competition for limited resources. Why argue over access to something as abundant as sunshine? Especially when it also tackles the other tragic consequence of our rush to depend so heavily on fossil fuels: climate change.
Solar power is a truly exciting field that is rapidly developing, and in the final pages we look briefly at emerging technologies on many different fronts (e.g. Figure 1.24). Globally, the growth of innovative solar technology is happening at a faster rate than any other energy technology. It needs to, because global energy demand is also rising quickly. The global growth of PV power installations alone is just about keeping pace with the global growth of energy demand. With the other solar technologies, it is possible that over the next 50 years solar power can meet a much higher percentage of demand and, together with the other renewable energy sources, gradually replace fossil fuels. When technologies dependent upon these fuels finally come to seem as archaic as a steam engine does to us today, we will know that we are finally living in the solar age.
References
BP, June 2009, ‘BP Statistical Review of World Energy 2009’, bp.com/statisticalreview Lindsey, Rebecca, 2009, ‘Climate and Earth’s Energy Budget’, NASA, available at http://
earthobservatory.nasa.gov/Features/EnergyBalance/
Loster, Matthias, 2006, ‘Total Primary Energy Supply: Required Land Area’, www.ez2c. de/ml/solar_land_area/
WEC, 2007, Survey of Energy Resources, www.worldenergy.org/publications/survey_ of_energy_resources_2007/solar/720.asp
heating and cooling in order to minimize the use of imported forms of energy. It means that elements of the building construction themselves collect solar energy and manage it efficiently. By contrast, active solar systems use added-on panels, collectors and stores, with pumps and fans. In practice, this distinction can become blurred and the two are often combined. However, keeping in mind the separate principles helps a designer minimize artificial energy use.
A certain amount of the sun’s heat and light needs to be allowed in and retained within the building. The quantity will vary throughout the year according to the seasons. Passive solar architecture – sometimes called ‘low- energy building design’ – should allow for the successful management of the internal lighting and air quality, the humidity and temperature, depending on the outside conditions. Many activities within a building create heat: our bodies give off heat, as does cooking, using hot water and electrical equipment, such as printers, computers and refrigerators. If managed properly, these sources of heat, together with that of the sun itself, could be all that is needed in order to maintain a thermally comfortable internal environment. A certain minimum number of air changes per hour should be permitted to keep the atmosphere fresh. Draughts or ventilation should be controllable in order to prevent overheating or cold temperatures, depending on the conditions outside.