In the 21st century, solar power has already become part of daily life. From solar heated swimming pools to sun powered homes, there are many examples that demonstrate the useful application of the clean, safe, sustainable power of the sun. As concern grows about the effects of burning fossil fuels, and the possibility of exhausting non-renewable energy sources, the future of solar energy looks bright. As of 2013, the technology is not without its problems, and so far, applications have mostly been relatively small scale, but a great deal of research is going on in this area, and there have been a number of very promising developments.

The Sun is potentially a huge source of renewable, clean energy. Some estimate that sunlight could produce 10,000 times as much power as the Earth used at the turn of the 21st century. There are, however, major technological challenges to be met in harnessing that energy effectively. There are a number of different technologies available, and under development, that use sunlight to provide power.

Sunlight can be used simply to heat water which is then used to provide central heating for homes. Alternatively, it can be used to generate electricity using photovoltaic (PV) cells arranged on solar panels. A third method is to concentrate sunlight on a target to generate heat, which may be used directly for industrial purposes, or to provide electricity.

These panels rely on the photoelectric effect, where certain substances produce a small electric current when exposed to light. Photovoltaic cells are constructed to exploit this effect, and solar panels consist of large arrays of these devices positioned to receive as much sunlight as possible. Together, they can generate significant amounts of electricity. Although, as of 2013, they are relatively inefficient, they normally have very low running and maintenance costs, and can be very effective in providing power for homes. A great deal of research is taking place into improving efficiency and constructing cells from cheaper materials.

Most solar panels consist of crystalline silicon PV cells, which are 18-24% efficient in converting sunlight into electricity. A number of alternatives, however, are under investigation. Thin film cells can be made from a variety of materials. Although these are currently less efficient than standard PV cells, they are lightweight, flexible, and cheaper to manufacture. Multijunction cells can achieve an efficiency of more than 43%. They are structured so that different parts of the cell are tuned to capture sunlight at specific wavelength ranges, rather than having a single receptor that misses a portion of the available energy.

Another promising area is the dye-sensitized solar cell (DSSC), sometimes called the Gratzel cell, after Michael Gratzel, who first developed it in the 1990s. These use a dye to capture solar energy and produce a flow of electrons, which are replenished via a liquid electrolyte layer below. Although potentially cheap to produce, they are only about 12% efficient, and there are problems with durability that may affect commercial use. For example, the liquid can freeze in cold conditions, or expand when it is warm, and may leak. Researchers have developed a version of the cell in which the troublesome liquid electrolyte is replaced with a solid material, opening the way for cheap and durable solar panels.

In addition to the development of cheaper and more efficient photovoltaic cells, an important part of the future of solar energy lies in the construction of new buildings and the retrofitting of many older ones. Some experts predict that most, if not all, new buildings will have solar panels installed on roofs. Since these are also easily fitted, many older buildings may receive upgrades to run on power from the Sun. Experts and environmentalists hope that green-energy building will be encouraged by governments through generous tax incentives, exemptions and grants for alternative energy use.

Solar panels on a roof can, in some areas, provide all or most of a home’s energy needs. In cases where people live in multi-storey accommodation, however, the amount of roof space is very small compared to the number of homes. While small, individual, applications can take some of the strain off the electrical grid, if the Sun is to provide the power needs of cities and industries, its future must lie in large solar powered electricity generating stations.

The largest problem facing the harnessing of solar energy using PV cells is the space required to build the power plants. A plant is comprised of thousands of solar panels, not unlike those currently installed on alternative energy homes. Because of this, they require a consistently sunny area and a considerable amount of space. Currently, the one of the largest power stations in the world covers more than 10 square miles (16.9 km2) and creates enough electricity to run about 200,000 homes. Some experts suggest that to provide power for the entire United States, an area approximately 100 miles (160.9 km) per side would be required, probably somewhere in the desert climate of the American Southwest.

A number of other possibilities exist for the large-scale harnessing of the Sun’s power. One example is concentrating solar power (CSP) technology. Rather than generating electricity directly, these concentrate sunlight to heat water, providing steam to drive a turbine that produces electricity just like a conventional power plant. They may consist of arrays of parabolic mirrors that focus sunlight on a linear tube filled with liquid. Alternatively, heat from the Sun may be focused by a parabolic mirror to heat a fluid that drives a Stirling engine, which provides the mechanical energy for electricity generation.

Another proven system is the “power tower,” in which an array of sun-tracking flat mirrors focuses heat from the Sun on a container of liquid that is used to provide steam for a generator. A number of plants are in operation, producing 10-20 megawatts of electricity. Future plants may deliver up to 200 megawatts.

One encouraging trend relating to the future of solar energy is that many of the world's greatest innovators are choosing to focus their talents and funds on improving alternativeenergy technology. Many award schemes — funded by various governments around the world — focus on providing solar power economically, and on a large scale. In many countries, citizens are given financial incentives for switching to “green” energy sources, and installing their own solar panels.

Although, as of 2013, there are many encouraging signs that governments are acknowledging the need for alternative energy sources, and promoting research into solar energy, the answer lies in part in the hands of the world's citizens. What ordinary citizens choose to buy and support will influence the trends of the future. By installing solar panels, donating to research organizations involved in alternative energies, taking a university degree in a related subject, and voting for measures promoting alternative energy development, anyone can have a say the future of solar energy.