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
