Uses of Solar Energy





Daylighting oculus at the top of the Pantheon in Rome. Image by Arnold Dekker.

Daylighting oculus at the top of the Pantheon in Rome. Image by Arnold Dekker. License: GNU FDL

Energy from the sun is the primary source of renewable energy. It creates wind for our windmills and rain for our hydroelectric reservoirs. To some extent, along with the moon, it causes the tides for tidal power. Arguably, it maintains the molten rock under our planet’s surface so it may even be involved with geothermal energy. This would mean that the Sun impacts all renewable energies.

Power from the sun has been utilized for thousands of years in architecture and agriculture. During the Little Ice Age farmers in Europe used walls erected next to fruiting plants to maintain heat in their gardens during the short growing seasons. Some examples of ancient architecture incorporate solar energy into their design for lighting or even passive heating purposes. Only recently has our focus shifted to the conversion of sunlight to electricity. Let’s go a bit further into the marvel that is our modern use of solar energy.

What are some ways solar energy is used?

Concentrated Solar Power (CSP)

Concentrated solar power. Image by afloresm.This form of solar energy relies on reflecting light from large areas onto a smaller point which drives a heat engine. This is similar to concentrated photovoltaics which reflect light onto a photovoltaic surface. Typically CSP systems drive a steam turbine to generate electricity but other heat engines like Stirling engines can be used. Concentrated solar power is also used for solar thermal energy collectors. These use a medium for storage of thermal energy (heat) from the Sun. The most common example being solar water heaters that store heat in water. Other mediums like molten salt and graphite are used for industrial scale solar thermal energy storage.

Building Integrated Photovoltaics (BIPV)

The CIS Tower, Manchester, England. Image by Pit-yacker.

The CIS Tower, Manchester, England. Image by Pit-yacker. License: CC BY-SA 2.5

Most people know about photovoltaic panels being mounted on a building. Building integrated photovoltaics takes this a step further by replacing components with photovoltaic integrated materials. Common forms of this are solar panel shingles designed to replace roofing shingles, thin film solar cells attached to roofing sheets, transparent photovoltaic glass for windows and skylights, and solar panel facades. These materials are more expensive than their traditional counterparts, even though they might pay for themselves over time, which has meant consumer uptake is slow. This technology is primarily seen in commercial buildings where investment in solar power can be seen as an added financial incentive to the building owners.

Solar Water Heaters

Passive (thermisiphon) solar water heaters in Jerusalem. Image by Gilabrand.

Passive (thermisiphon) solar water heaters in Jerusalem. Image by Gilabrand. License: GNU FDL

Solar water heaters fall into two major categories, passive and active. Among passive systems the most basic is Integrated Collector Storage where the sunlight collector (usually a flat panel filled with water) and water storage tank are combined into a single unit. Imagine a tank of water suspended over a mirror reflecting sunlight onto the tank. One of the most common passive systems is Convection Heat Storage also known as thermosiphon solar water heaters. These systems separate the collector and position it below the water tank. As the collector heats the water it rises into the water tank and is replaced by colder water from the tank. Active solar water heaters add a pump to allow for the collector to be placed above the water tank. Direct active solar water heaters will simply pump water from tank to collector and back. Indirect active solar water heaters use a heat transfer fluid to heat the water. Both of these systems require a controller as well to regulate the pump. This has the advantage of being able to prevent both overheating and freezing which passive systems are vulnerable to.

Passive Solar Building Design

Solar Umbrella House in Venice, CA. Image by CalderOliver.

Solar Umbrella House in Venice, CA. Image by CalderOliver. License: CC BY-SA 3.0

It’s possible to design a building to maximize the amount of solar energy gained in the winter and minimize the amount gained in the summer. This is the basis of passive solar building design which uses structural design to get the most out of solar energy. Everything has to be considered including insolation rates, window placement, construction materials, and much more. Some of these systems do use active solar components like solar water heaters and photovoltaics particularly to power electronics to drive things like shutters, air vents, and other mechanical components to optimize the internal climate as the solar environment changes externally. These components are intended to require minimal power and are only to change the flow of solar energy impacting the building.

Solar Powered Vehicles

Helios solar powered plane in flight. Image by NASA.

Helios solar powered plane in flight. Image by NASA. License: US PD

The use of photovoltaics to power an electric motor is at the heart of solar powered vehicles. Some vehicles are designed to run purely from photovoltaic electricity with no batteries or other energy storage. These are typically involved in competitions or record setting attempts. Commonly batteries are implemented to allow for continuous operation during low light situations. Automobiles, planes, boats, air balloons, and even space ships have all been run off solar power. There is also a category of vehicle integrated solar panels used to power axillary systems like air conditioning lessening the load on the primary engine or motor.

Solar Water Treatment

SODIS water disinfection in Indonesia. Image by SODIS Eawag.

SODIS water disinfection in Indonesia. Image by SODIS Eawag. License: GNU FDL

Humans have been using solar energy to desalinize brackish or salt water for hundreds of years. Using the thermal energy provided by sunlight to evaporate water drives solar water distillation. This is a technique that is used in “backyard” distillation and industrial sized solar desalination projects that provide fresh water to entire cities. In areas lacking potable water due to water born illness solar disinfection can be done using plastic water bottles exposed to sunlight. This is a very cheap, easy, and safe way of sanitizing water using solar power and plastic making it perfect for rural areas and developing countries where more sophisticated water treatment is unavailable.

Solar Ponds

Solar evaporation ponds in Atacama Desert. Image by NASA.

Solar evaporation ponds in Atacama Desert. Image by NASA. License: US PD

A useful solar energy technology in rural and developing areas is the solar pond. These are simply a pond of salt water which naturally creates a vertical salinity gradient that increases with depth. This counteracts the temperature gradient caused by solar energy which decreases with depth. Decreasing density gradient due to temperature differences is why hot water rises to the surface. However in salt water this action is counteracted by the previously mentioned salinity gradient. The result is a pool of water where solar thermal energy becomes stored in the water at the bottom rising to temperatures as high as three times the surface temperature. Ultimately this hot water can be used to generate electricity using heat engines or for industry.

Greenhouses

Large commercial greenhouse in Illinois, USA. Image by Bjvw006.

Large commercial greenhouse in Illinois, USA. Image by Bjvw006. License: GNU FDL

Agriculture has been benefiting from greenhouses for thousands of years. Normally incoming solar energy hits the ground and is both absorbed and reflected. Some of this absorbed energy is radiated back as thermal (heat) energy which then rises through the atmosphere. The purpose of a greenhouse is to use infrared transparent glass to allow solar energy through which is then trapped within as thermal energy. There are many benefits to the greenhouse effect for agriculture. Plants are shielded from external cold, transparent windows can be covered to protect from heat, and humidity levels can be carefully controlled to fit certain crops. This temperature control means longer growing seasons sometimes allowing for year round produce.

Solar Updraft Tower

Solar updraft tower prototype at Manzanares, Spain.Solar updraft towers are a novel approach to using solar energy to generate electricity. They also provide greenhouses for agriculture making use of convection, the green house effect, and the chimney effect. Essentially a large cylinder is built with an open top and bottom. At the bottom is a large solar collector area like a green house that circles the base of the cylinder. This area is open to airflow from the sides. When sunlight passes through the collector glass it heats the ground which gradually raises the temperature of the air inside. This air rises up the chimney drawing in fresh air from the sides creating a convection cycle. As this air flows into the chimneys it passes by several wind turbines that generate electricity.

By now you should hopefully understand that solar energy goes a lot farther than charging your calculator. Aside from being a top contender for large scale electrical generation solar power is impacting people all around the world. Researchers, architects, designers, humanitarian groups, and even consumers are all pushing the limits of what we can do with the power of sunlight.

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