L2 - Solar Energy Flashcards
Technologies of Solar energy
- Solar heating (active & passive heating)
- Solar thermal cooling
- Solar thermal engines/electrcity
- Solar photovoltaics
How much solar radiation arrives on earth?
- Arrives on earth at a max flux density of 1 kw/m2
- Wavelength band 0.3 - 2.5 micrometre (short wave radiation)
How much solar energy reirradiated back from the earth?
- Flux density same as arrives (1 kw/m2)
- Wavelength band longer at 5-25 micrometre
How does sun radiation flux vary in habited areas?
- Varies from 3 - 30 MJ/m2 per day depending on:
- Place
- Time
- Weather
Difference between direct and diffuse radiation
- Direct is a direct beam from the sun
- Diffuse is when the sun beam hits cloud and dust and is diffused out
How does availability of solar radiation vary
- Varies hourly (higher midday)
- Location on the globe: latitude (higher in equator etc)
- Varies seasonally
Types of solar collectors
- Unglazed panels
- Flat plate water collectors
- Flat plate air collectors
- Evacuated tube collector
- Line focus collectors
- Point focus collectors
Unglazed panels
- Surface is black to absorb as much solar
- Most suitable for raising heat by a few degrees (eg swimming pool)
Flat plate water collector
- Most commonly used in solar thermal systems for solar water heating
- Surface of the absorber plate is black to absorb as much solar. Very dark with high absorptivity in visible region and low emissivity in long-wave infrared to cut losses of temperature
Flat plate air collector
- Not very common
- Used for space heating only
- Can combine with photovoltaics panel to provide both heat and elec
Evacuated tube collector
- Absorber plate is a metal plate down centre of each tube
- Absorber plate uses heat pipe to transfer collected energy to water circulating along a header pipe
Line focus collectors
- Mainly used to generate steam
- Consists of a trough which can be pivoted to face the sun
Point focus collectors
- Used for steam generation or driving stirling engines
- Can track the sun in two dimensions (up&down, side to side)
Which type of solar collector is most efficient?
- Unglazed panels most efficient (80%) however loses efficiency quickly once the temperature rises (T collect - T ambient difference higher)
- Flat plate (most common) has slightly lower efficiency (75%) but can maintain efficiency better at higher temp diff
- Evac tube best at maintaining efficiency at high temps
Solar heating technologies
- Solar domestic hot water heating
- Combisystems
- District heating
Solar domestic hot water heating (SDHW)
- Either forced circulation or thermosiphon
- Designed to deliver 100% hot water requirements in summer
- 40-80% hot water requirements annually
- Include a supplementary heater (eg integrated electric or gas heater) or thermosiphons?
Thermosiphons (how it works, benefits and disadvantages)
- Use gravity to circulate heat transfer medium (normally water) between collector and tank
Benefits: works without pump and controller. highly efficient if designed well
Disadvantage: tank must be located above or beside collector
Forced cirulation (how it works, benefits and disadvantages)
- More common than thermosiphon
- Tank can be instaleld anywhere as heat transfer is circulated by a pump
Benefits: integration with other systems easier. Tank does not have to be located on the roof
Disadvantage: higher flexibility comes with higher complexity. Needs sensors, controllers and pump
Combined DHW and space heating (combisystems)
- Provides both hot water and space heating (radiators)
- Collector size range 7-20 m2 and tanks in the range of 300-2000 litres.
- Often more complex than SDHW providing hot water only
- Easier to have block heating applications and integrate into other systems
Solar heating: passive heating
- Designing buildings so that they absorb energy from the sun without you having to add external energy inputs.
- Reduces the energy required to warm the habitable areas
Solar thermal cooling
The production of cold air from a heat source
Solar cooling: solar energy absorption
- Most commonly used type of cooling system
- Cooling power is several hundred kW. Normally large scale
- Devices powdered by heat from district heating network, recovery heat or cogeneration heat
- Temp required 80 for 1 stage operation, >140 for 2 stage
- Coefficience of performance COP 0.6-0.8 (1 stage), 1.2 for 2 stage
Advantages & disadvantages of summer cooling with solar energy absorption
Advantages:
- Long life span (no moving parts in comrpessor)
- Low noise production (absense of vibrations)
- Reduced demand for electrical energy
Disadvantages:
- High power demand
- Not high COP
Solar thermal engines and electricity generation technologies
- Solar power tower
- Trough concentrator
- Solar dish
- Solar chimney
- Solar pond
Solar power towers
Solar thermal engine tech
- 115 m tower surrounded by 600 reflectors (heliostats)
- Heliostats track sun and direct its rays to heat exchanger (receiver) at the top of the tower
- Receiver converts concentrated solar energy into steam
- Steam is stored in tanks and used to drive turbines to produce electricity for up to 6000 homes
- Has storage capacities
Trough concentration
Solar thermal engine & electricity generation tech
- Parabolic trough tech is most proven solar thermal electric tech.
- Efficiency is around 21% for conversion of direct solar radiation into grid electricty
Solar dish
Solar thermal engine and electricity generator
- Mirrored dishes focus and concentrate sunlight into a receiver mounted at the focal pointof the dish.
- Quite expensive, not commonly used
- Doesnt have storage option
Solar dish
Solar thermal engine and electricity generator
- Mirrored dishes focus and concentrate sunlight into a receiver mounted at the focal pointof the dish.
- Quite expensive, not commonly used
- Doesnt have storage option
Solar chimney
Solar thermal engine and electricity tech
- Sun radiation used to heat a large body of air under an expansive collector zone.
- Hot air rises as a hot wind through large turbines to generate elecc
Does not use water but has a large dimension
Solar pond
Solar thermal engine and elec tech
- Contains 3 layers:
- top layer is cold with little salt content
- bottom layer is hot with high salt
- separating these layers is gradient zone. Here salt content increases with depth. Water in gradient cant rise because water above it has less salt so is lighter
In the bottom layer, heat can be stored and withdrawn
Solar photovoltaics (solar PV)
Not thermal. Directly converts sunlight into electricity.
- Most successful solar electricity generator in global deployment
How do PV cells work
PV cell materials - semiconductors (eg. crystalline silicon) are doped with impurities to create surplus of free electrons for one layer and their deficite for the counterpart layer.
When there is no sunlight, an equilibrium will be reached.
Generations (types) of PV cells
- 1st generation (1970s): monocrystalline silicon p-n junction, expensive manufacture process
- 2nd generation: polycrystalline silicon - thin films, cheaper but less efficient due to less purity
- 3rd generation: a whole range of new tech including non-semiconductor (polymer cells, nanoparticles
PV cell applications
- Residential
- Non-domestic
- Power plants
- Remote power
Positive environmental impacts of solar thermal and PV
Benefits
Positive environmental impacts of solar thermal and PV
- Reduction of emissions of greenhouse gases (CO2, CH4, N2O) and prevention of toxic gas emissions (SO2 particulates)
- Recovery of degraded land
- Potential reduction of the required transmission lines of electricity grids
