# Solar energy

Solar Energy systems come in a diverse range of technologies and are largely divided up into two main categories which are solar electric and solar heating. Solar electric really covers solar photovoltaic cells used for generating electricity while solar heating systems typically are used for heating water.
A further subdivision could be thought of as active and passive systems. Active systems are those which are built solely to capture solar energy whereas as passive systems are more generally design features that make use of the available solar energy. An example of the latter is building architecture.

## The Resource

The Solar Resource
At the top of the atmosphere the solar irradiance on the Earth is 1,366 watts per square meter (w/m-$^2$). This means that the total solar energy received by the Earth is:
pi * radius$_{Earth}$$^2$ * 1,366 (w/m-$^2$)
Plugging in the values this gives:
or rounding off this number it is: 1.74 * 10$^{17}$ watts or 170,000 TWh where a terrawatt is 1 * 10$^{12}$ watts
Clearly this value is not exact because the Earth is in an eliptic orbit and so the amount of solar energy varies slightly throughout the year and also the Earth bulges ever so slightly. But these effects will not change the overall magnitude of the figure.
What matters though is the amount of solar energy hitting the ground at any point and this varies depending on the latitude, the season, the local time and the thickness of the atmosphere that it has to travel through. In general at least 30% of the 1.36 Kwh is lost in the atmosphere while at higher latitudes where the sunlight has to pass through more atmosphere because it is entering at an angle it can probably be as high as 50%. The effect of season is most pronounced at higher latitudes.
A point to note about local time, is that mid-day local time is generally defined as the time at which the sun reaches the highest point in the sky at that location. In reality then there is just a few hours in the day when the sun is around it's maximum angular height in the sky and supplying most energy

## The Vision

It has long been the dream of many solar proponents that solar energy can provide all our energy needs. In 2004, the worldwide energy consumption of the human race was on average 15 TW (= 1.5 x 10$^{13}$ W). The figures above show that the Earth receives more than 10,000 times this amount of energy and show clearly there is enough to provide all our energy needs.
It is hard to argue with these figures and it has been suggested many times that the one off gift of nature of fossil fuel energy should be used to build the solar energy infrastructure which can last humankind indefinitely given that other problems like ecological collapse, global warming and a pollution crisis are avoided.
A popular calculation that has been done is to determine how many square kilometers would need to be covered by solar energy to capture enough energy for our needs. The figure widely quoted is a box about 100 miles (or 160 km) per side which is 10,000 square miles (25,600 km$^2$) but this figure is just for the United States.
The calculation is derived roughly as follows. It is assumed that the location is at a relatively low latitude in sunny place like the a desert and that 1000 w/m$^2$ are received at ground level. Taking current solar photovoltaic cell efficiencies actually deployed, we assume a value of around 10% efficiency. Therefore to achieve 15 Twh (10$^{12}$w):
Taking the square root and converting from m$^2$ to km$^2$ gives:
387,298 metres or 387 km per side. Doing the calculation using an efficiency of 15% instead of 10% gives a value of 316 km (~197 miles) which is considerably lower. And an even higher efficiency of 20% which would be less likely for such a large system would reduce this to 274 km.
If this calculation was done for just the total global amount of electricity used then it would be lower. Other factors to consider are that by introducing more efficient energy technologies and greatly increasing the amount of public transport would greatly reduce the amount of oil and thereby total energy usage.

## The Needs
# Technologoies

Solar Energy Technologies

## Solar Electric

The generation of electricity from solar energy is dominated by the use of solar photovoltaic cells. However there is the less well known solar thermovoltaic effect which uses heat instead to convert solar infrared light into electricity directly.

### Solar Photovoltaics

These convert sunlight into directly electricity. For a full discussion of this process see [[[w:Solar_Cell]]].
There are three main types of solar cells. These are crystalline silicon solar cells, amphorous silicon solar cells and thin film solar cells.

#### Crystalline Solar Cells

These were the first types of solar cells and require a very pure silicon crystal to be grown from which the solar cell is manufactured. The disadvanages are that the crystals are time consuming and require a lot of energy to grow them. This is because the silicon is held at a high temperature. Indeed this is one of the reasons why it has been so difficult to rapidly increase global production of crystalline solar cells.
As a result of this, the energy payback time of crystalline solar cells has been typically 10 to 20 years, although this is being reduced slowly as the technologies improves and the cells are being manufactured using thin slices of silicon.
One of the main advantages though of crystalline cells is that they tend to have the highest efficiencies of all solars and over the past decade or so, figures from 25% to just above 30% have been achieved. Typically values in the field are less.

#### Thin Film Solar Cells

Thin Film solar cells cover a diverse range of cells types using a number of different chemical elements. For a full discussion of this see w:Solar_Cell#Thin_films.
The main advantages of thin film cells is that their energy payback times tend to be lower and they tend to be much more flexible and less brittle. Some types can be even bent and rolled up as sheets.
Thin film cells currently cover about 10% or less of total global production of solar cells.
One of the less discussed problems with thin film solar cells is that they tend to use both relatively rare and toxic chemicals which could present longer term environmental problems.

#### Concentrating Systems

Silicon crystalline solar cells can be used in conjunction with solar concentrates to increase the amount of sunlight following on a given cell. Some types of solar cells even achieve slightly higher efficiencies by doing this.
The main advantages of employing this technique is that you can use much more expensive and higher effeciency cells and at the same time use collect a bigger area of sunlight using cheaper components and thereby reducing your overall cost.

### Solar Thermovoltaics

These type of solar cells operate by converting infrared light into electricity. They are effectively the same as solar photovoltaics except they are just operating at a longer wavelength. The sun produces a significant amount of infrared radiation as to any hot objects like furnaces. In theory these cells can also be used to operate off the infrared emitted by furnaces.

### Solar Tower

These are still largely in the experimental phase. They are often referred to as solar updraft tower because they consist of a canopy over a relatively large area under which the area is heated. The canopy is angled upward towards the base of the tower and this allows the heated air to flow under convention up the tower. Significant updraft is created and this can be enough to drive a turbine in the tower shaft.

### Solar Stirling Engine
## Solar Heating and Cooling

Solar Heating can be further classified into active and passive systems. Examples of active systems are solar water panels that you often see on roofs for heating water, whereas a south facing building that takes advantage of the sun can be considered as a passive form of solar energy.
Solar heating and cooling is the less glamorous side of solar energy compared to the more high tech solar photovoltaics. However it is an important area because significant amounts of energy are used for heating water and for heating and cooling buildings. In the USA, electrical demand is actually higher in the summer rather than the winter because of the amount of electrical powered air-conditioning use. There are considerable opportunities for replacing and reducing the amount of fossil fuel energy to supply these services.

### Solar Panels

Solar panels are now used quite extensively domestically especially in Mediterranean countries and Israel for heating water.
They basically consist of a radiator painted black to absorb the sunlight and covered by glass to trap the heat. The water is circulated through the narrow piping so that it has a relatively long distance to travel and therefore gives it time to pick up the heat.
At present there is a huge range of panel designs and manufacturers. Solar panels are the ultimate for the do it yourself (DIY) enthusiast because anyone with basic plumbing skills should be able to put together a relatively simple system. Vacuum tube seems to offer the best solution as a reflector is placed under them and this enables 360 Degree sunlight around the tube effectively using the whole tube while the sun shines. Tempreture of 303 Degree celcius has been measured of the inner copper tip which is used on high pressure systems. The low pressure system has no inner copper tube however tempreture of 75 degree has been measured. The time is now to make use of natural elements and elimanate destroying the planet. Lets use this technology its free.

### Solar Chimney

A solar chimney is a way of improving the natural ventilation of buildings by using convection of air heated by the sun.
See w:Solar_chimney

## Passive Solar Energy
### Passive Solar Lighting
#### Sky Lights
#### Solar Tubes
# Generation of Electricity

Generation of Electricity by Solar

## Utility Factor

The utility factor refers to the fraction of time in a day that can be used to generate solar power. Clearly average over a year, it is going to be a theoretical maximum of only 50%. In practice this value will be even lower because if you account for the period around both sunrise and sunset when the sun is at a very low angle in the sky, there will not be sufficient power from the available light to generate any significant amounts of power. Taking this into consideration then would suggest a value of 40% being more typical.
It might be worth noting that the typical utility factors achieved for wind power so far are around 26% on average with the best sites as high as 35%.
Utility factors for fossil fuel power stations burning coal, gas, oil and nuclear power stations can be as high as 80% to 90% since other than maintenance there are no other limitations. In situations of a severe drought though, the availability of fresh water for cooling can impact this.
Therefore to replace a given electricial capacity of a fossil fuel power station typically requires double the capacity when being replaced by a solar electrical plant. So for example to replace a 500 MW coal plant would require an installation of 1000 MW (1GW) of solar.

### Capacity and Energy Usage

When discussing any energy supply it is important to always distinguish between figures for installed capacity and actual energy produced. One should look out for units of wattage which are usually quoted for capacity and kilowatt hours (Kwh) which are used for power produced.
In a traditional fossil fuel power station, it typically might run at close to full power at its utility factor. Its yearly power output will then be approximately:
For a 500 MW power station this is: 365 * 24 * 0.8 * 500 = 3,504,000 MWh = 3.5 TwH
A figure worth remembering is that there are approximately 8,760 hours per year.

## Residential Power

The generation of electricity by solar photovoltaics for residential use has been growing reasonably steadily over the years. It is still at a very small level, certainly far less than 1% of domestic electrical consumption.
For a residential setting the quantity of power needed varies but it ranges from about 2Kw to 4Kw systems. They can be setup as grid connected or offgrid. For offgrid systems a battery bank is needed for storing power and this can add to the cost.

## Readings and learning resources
### Wikipedia

* Dye-sensitized solar cells
* List of photovoltaics companies
* Agrivoltaics