Solar - Tutorial Flashcards
What are the main atmospheric effects on solar irradiance and how is the
solar spectrum modified as it passes through the atmosphere?
Radiation passing through the atmosphere is absorbed, reflected, scattered and transmitted directly
Latitude is a big determinant of the pattern and amount of energy available
The tilt (or declination) of the Earth’s axis creates substantial variations in irradiance and distinct seasons
Reduced ultraviolet (UV) light due (mostly) to ozone in upper atmosphere (λ < 300nm) .
Water vapour and CO2 absorption bands in infra red (IR) region (λ > 700nm).
Briefly distinguish direct, diffuse and global solar radiation.
Radiation passing through the atmosphere it is absorbed, reflected, scattered and transmitted directly.
At ground level the overall global
irradiance has two main components:
Direct irradiance straight from
the Sun’s disc.
Diffuse irradiance coming from all directions in the sky
Describe the components and operation of a simple solar thermal system.
Solar energy is collected and used to heat water.
Consist of a dark surface behind glass that gets hot in the sun.
Simplest device is the ‘flat-plate’ collector employing glass plate and insulation.
Consists of black absorber plate, insulation, casing, transparent cover and pipe for water flow.
Evacuated tube collectors are more
efficient, complex and expensive.
Briefly describe and compare the main approaches for solar hot water collectors.
Basically the more complex collectors – evacuated tubes -have higher efficiencies but have more components and are more complex to manufacture as well as being relatively fragile. These all raise costs. The simplest – unglazed flat plate – is robust with few components but performs poorly
What advantages has a concentrating solar collector over conventional collectors?
Concentration allows much higher incident beam intensity which can achieve high temperatures as well as needing a much smaller collector. Tend to be more complex and costly
What are the fundamental energy conversion limits associated with solar water heating and solar thermal generation?
For solar water heating these can be 100% efficient but this implies a system that has a collector temperature at ambient – it would 100% ineffective! Typical efficiencies can be fairly high. Solar thermal generation is limited by 2nd law of thermodynamics and unless the heat source is raised to a very high temperature efficiencies will be poor.
What are the environmental impacts associated with solar thermal generation?
Large land requirement which limits use. Water for washing concentrating devices – problematic in deserts. Transmission impacts.
List and briefly characterise the main PV technologies.
Mono- and poly-silicon (15-20%), Gallium Arsenide (20-28%), thin films, Cadmium Telluride and the rest.
Describe the structure and operation of a single junction photovoltaic cell.
PV cells converts solar energy into electricity by exploiting the photovoltaic effect where voltages or currents within materials change as sunlight falls upon them.
The process relies on the properties of semiconductors
such as silicon
Conventional PV cells use a junction (or interface) between two very thin dissimilar semiconducting materials. Pure silicon is electrically very stable so other materials are added – a process called doping.
n-type silicon is doped with phosphorous (~103:1 Si:Ph) to provide surplus electrons and an overall negative charge. p-type silicon is doped with boron (~107:1 Si:B) to provide
deficit of electrons (holes) and an overall positive charge. The combination of these materials creates a ‘p-n’ junction and an electric field in the vicinity of the junction. This causes negative charges particles (electrons) to be drawn in one direction and vice versa.
A photon of light striking the silicon penetrates and may transfer its energy to an electron. This will ‘excite’ the electron freeing it to move around within the material and leaving behind a ‘hole’. Although electron moves randomly,
the electric field will draw it towards
n-type side. On aggregate this current can power external electric loads – metal
contacts facilitate this. Producing power also requires a voltage – this is created by the p-n junction electric field.
What are the standard test conditions for PV panel testing?
Insolation at cell surface of 1000 W/m2, 25°C and Air mass of 1.5.
Outline the components of a simple grid-connected domestic PV installation.
- PV array
- DC wiring between array and inverter
- Inverter provides power electronic conversion of DC to
50 Hz AC and - Maximum Power
Point Tracking (MPPT) - AC wiring to isolator and PV meter
- System may include storage, back-up generation and DC
loads
What are the main environmental impacts associated with solar PV?
Some visual impact especially for field sized farms. Rooftops largely invisible. Production tends to be where impacts occur particularly use of toxic chemicals (Cadmium), as well as high energy consumption. Production process much improved and plenty of scope to reduce impact further.
