APES 6.7-6.8: Solar and Wind Energy

Thermal mass

A material's ability to absorb and store thermal energy, often used in passive solar buildings. Stone, cement, and water can absorb heat during the day and release it as the surroundings cool.

How does thermal mass reduce a building's energy demand?

It moderates temperature fluctuations by storing excess heat and releasing it later. This reduces the need for auxiliary heating and cooling.

What quantity of heat is required to raise the temperature of a substance in a solar thermal system?

The heat transfer can be calculated with $q=mc\Delta T$, where $m$ is mass, $c$ is specific heat capacity, and $\Delta T$ is the temperature change.

Solar energy

Radiant light and thermal energy emitted by the Sun that can be captured or converted into useful forms of energy, including heat and electricity.

How are solar technologies classified as active or passive?

Active solar technologies use equipment such as photovoltaic cells, pumps, fans, collectors, or concentrators to capture and convert sunlight. Passive solar technologies use building orientation, thermal mass, shading, daylighting, and natural air circulation without powered equipment.

What fraction of incoming solar radiation is reflected by Earth, and what happens to the remainder?

About 30% of incoming solar radiation is reflected back into space. Most of the remaining radiation is absorbed by the atmosphere, oceans, clouds, and land.

Insolation

The solar radiation received by a surface, commonly reported as power per area such as $\mathrm{W\,m^{-2}}$ or daily energy per area such as $\mathrm{kWh\,m^{-2}\,day^{-1}}$.

What portion of the electromagnetic spectrum makes up most sunlight reaching Earth's surface?

Most surface solar radiation is in the visible and near-infrared regions, with a smaller amount in the near-ultraviolet region.

How does solar radiation drive Earth's water cycle and atmospheric circulation?

Solar energy evaporates water, and the warm, moist air rises by convection. At higher, cooler altitudes, condensation releases latent heat, strengthening convection and contributing to winds and storms.

What is the approximate global incoming solar power at the top of Earth's atmosphere?

Earth receives approximately $174\ \mathrm{PW}$ of solar radiation at the upper atmosphere, where $1\ \mathrm{PW}=10^{15}\ \mathrm{W}$.

What is the relationship between photon energy and wavelength?

Photon energy is inversely proportional to wavelength: $E=\dfrac{hc}{\lambda}$. Therefore, shorter-wavelength radiation has higher-energy photons.

Why does the amount of solar energy usable by humans differ from the total solar energy reaching Earth?

Usable energy is limited by location, time of day and year, cloud cover, atmospheric absorption, geographic conditions, and the amount of land available for energy systems.

How does photosynthesis store solar energy?

Green plants absorb light and convert it into chemical potential energy in molecules such as carbohydrates. This stored biomass provides food and wood and ultimately contributes to the formation of fossil fuels.

Why is solar energy considered renewable?

The Sun continuously supplies radiation on human timescales, making the resource effectively inexhaustible compared with finite fossil-fuel reserves.

Solar thermal energy

The use of sunlight to increase the thermal energy of a material, commonly for water heating, space heating, cooling, cooking, industrial process heat, or electricity generation.

How can solar thermal systems heat water?

Solar collectors absorb radiation and transfer the resulting thermal energy to water or another fluid. Common collector types include evacuated-tube collectors, glazed flat-plate collectors, and unglazed plastic collectors.

Solar chimney

A passive ventilation system consisting of a sun-warmed vertical shaft. Heating the air in the shaft causes it to rise, creating an updraft that draws air through the building.

How does building orientation affect passive solar heating?

Orienting windows and major surfaces toward the equator-facing direction generally increases winter sunlight and heat gain. Shading devices and vegetation can limit unwanted summer heating.

Why are deciduous trees useful in passive solar design?

Their leaves shade a building during summer, while their leafless branches allow more sunlight to reach it during winter. Their placement must be chosen carefully so winter solar gain is not excessively blocked.

How do solar cookers differ in their operating temperatures and light requirements?

Box and panel cookers typically reach about $90$–$150\ ^\circ\mathrm{C}$ and can function under partly cloudy conditions. Reflector cookers concentrate direct sunlight, can exceed $315\ ^\circ\mathrm{C}$, and require repositioning to track the Sun.

How do concentrating solar technologies generate high-temperature process heat?

Mirrors or other reflectors concentrate sunlight onto a smaller area, increasing the energy delivered per unit area and heating a working fluid or material for industrial use.

What is an evaporation pond, and how does it use solar energy?

An evaporation pond is a shallow basin in which sunlight and wind evaporate solvent, leaving dissolved solids concentrated. Applications include salt production, brine concentration, and treatment of some waste streams.

Unglazed transpired collector

A perforated, sun-facing wall that absorbs solar energy and preheats air as it passes through the wall. These collectors can raise incoming air temperatures substantially for building ventilation.

How can solar distillation produce potable water from saline water?

Solar energy evaporates water while leaving salts and many impurities behind; condensation of the water vapor produces distilled water. The process separates substances based on differences in volatility.

Solar water disinfection (SODIS)

A treatment method in which water-filled plastic bottles are exposed to sunlight for hours or longer. Solar ultraviolet radiation and heating help inactivate microorganisms, although the required exposure time depends on weather and climate.

How does solar energy treat water in a stabilization pond?

Sunlight supports biological activity, including photosynthesis by algae, which can consume dissolved carbon dioxide and help treat wastewater. However, excessive or toxic algal growth can make the water unsuitable for use.

Molten-salt thermal energy storage

A concentrated solar power system can heat molten salt and store the salt's thermal energy in insulated tanks. The stored heat can later produce steam and electricity when sunlight is unavailable.

Why are nitrate-based molten salts useful for thermal energy storage?

Mixtures containing sodium nitrate, potassium nitrate, and calcium nitrate are nonflammable, relatively nontoxic, and effective as heat-transfer fluids. Their high heat capacity and high operating temperatures make them useful for storing concentrated solar heat.

How does stored molten salt generate electricity?

Hot molten salt transfers energy to water in a heat exchanger, producing superheated steam. The steam expands through a turbine, converting thermal energy to mechanical energy and then electrical energy.

Photovoltaic effect

The generation of an electric current when light is absorbed by a material, such as a semiconductor in a solar cell. Photovoltaic systems convert sunlight directly into electricity.

How do photovoltaic systems differ from concentrated solar power systems?

Photovoltaic systems convert light directly into electrical current. Concentrated solar power systems use mirrors or lenses to produce high-temperature heat, which commonly drives a steam turbine or is stored thermally.

Why do concentrated solar power systems use mirrors, lenses, and tracking mechanisms?

They direct radiation from a large collection area onto a smaller receiver, increasing the energy flux and temperature. Tracking keeps the collector aligned with the changing position of the Sun.

What energy transformations occur in a concentrated solar steam plant?

Radiant energy becomes thermal energy in a working fluid, thermal energy produces steam, and the expanding steam becomes mechanical energy in a turbine. The generator then converts mechanical energy into electrical energy.

Why is energy storage important for solar electricity?

Solar input varies with the time of day, season, cloud cover, and weather. Storage allows energy collected during periods of sunlight to supply electricity later, improving reliability.

What factors can damage concentrating solar power equipment?

Dust storms, hail, and other severe weather can damage the reflective or transparent surfaces used to focus sunlight. Protective structures can reduce damage but may also block some incoming radiation.

What is the main distinction between supply-side and demand-side solar technologies?

Active solar technologies increase the supply of usable energy and are supply-side technologies. Passive solar design decreases the amount of external energy needed for heating, cooling, lighting, or ventilation and is therefore demand-side.

How can solar architecture reduce heating and cooling requirements?

Proper orientation, compact building geometry, shading, daylighting, natural ventilation, and thermal mass can maintain comfortable temperatures with less mechanical heating or cooling.

Why does the surface temperature of a material increase when it absorbs solar radiation?

Absorbed radiation transfers energy to the material's particles, increasing their average kinetic energy and therefore its temperature. The temperature increase depends on the absorbed energy, mass, and specific heat according to $q=mc\Delta T$.