Renewables

This lecture and next

Hydroelectric power

Solar energy

Wind turbines

Wave &Tidal

Geothermal

Example problems

Hydroelectric power

Hydroelectric power

Gravitational energy -> flow -> turbine -> El.

Electric power production: P ghqk

P : Power (W): density (998 kg/m3)

h : height (m)q : flow rate (m3/s)g : gravity acceleration (9.8 m/s2)k : efficiency coefficient

Example

What is the power generation of a dam operating with a flow rate of q = 100 m3/s, with a net head (height) of h = 150 m. The turbine is 85 % efficient.

P ghqk

P : Power (W): density (998 kg/m3)

h : height (m)q : flow rate (m3/s)g : gravity acceleration (9.8 m/s2)k : efficiency coefficient

P = 998 x 9.8 x 150 x 100 x 0.85 = 124,700,100 W = 124.7 MW

Units: kg m2 s-3 = W

Hydroelectricity

No CO2 emission

Most widely used renewable (2.3% of total)

Competitive: low cost per kWh.

Environmental impact: Changes river system. Destroys local habitats. Areas submerged.

Three Gorges Dam Worlds largest power station: 22.5 GW capacity.

Avoids 100 million tonnesCO2 emission by replacing coal.

Solar Photovoltaics(PV)

Generates electricity directly from solar radiation.

Third most installed renewable (after hydro and wind).

Total installed capacity: 140 GW

36.8% annual growth

The photovoltaic effectSemiconductor material (traditionally silicon)

pn-junction: separates negative and positive charges generated by photons.

Charge build-up generates a voltage

Current flows in external circuit.

PV systems

PV cell PV panelPV system

PV farm

Efficiency: typical 15 20 %

Example 1

On a sunny summer day the solar irradiance is I = 1 kW/m2. What is the power output of a 10 m2 PV array on the roof of a house? The efficiency

of the PV array is 12%

P = AI = 0.12 x 10 x 1 = 1.2 kW

Regulator and conversion efficiencies also factor in

Example 2

Annual solar energy output: E = A H PR

whereE= Energy (kWh)A = Total solar panel Area (m)

= solar array efficiency (%)(at standard test conditions)H= Annual average solar radiation on tilted panelsPR = Performance ratio, coefficient for losses (range between 0.5 and 0.9)

Calculate annual solar energy output for the module in example 1, when H= 1000 kWh/m2, and the performance ratio is PR= 0.75

E = 10 x 0.12 x 1000 x 0.75 = 900 kWh per year

Solar thermalSolar energy produces heat for domestic hot water (DHW) or space heating.

Low grade energy for low grade energy requirement

Solar thermal flat plate collector

Solar radiation heats absorber plate

Heat carrier fluid (water or air) transports heat to heat reservoir.

Flat panel heat output

Heat output: Q = (Absorbed energy heat loss) x Area

Heat output: Q = [ I U(Tp Ta)]A

where: transmittance of cover plate: absorbtanceof absorber plate

I : solar irradiance (kW/m2)U : heat loss coefficient (kW/(degC m2)Tp: Mean absorber temperature (degC)Ta : ambient temperature (degC)A : collector area (m2)

What is the efficiency?

Collector efficiency vs. operating temperature

Concentrated solar power

Parabolic dish Parabolic troughPower tower

Sunlight is concentrated by mirrors or lenses to a small absorber area and converted to heat. A heat engine converts the heat to electric power.

Solar energy

+ Renewable+ No pollution (except manufacture and transport)+ Can be used in remote areas+ Surplus energy can be sold to grid

- Intermittent supply. Requires storage- Cost of solar cells high (but decreasing rapidly)- Not equally distributed energy

Solar Energy

Photovoltaics: Solar energy converted directly into electricity. Internal electric field separates mobile charge carriers generated through absorption of photons.

Solar heat: Solar energy converted to low temperature heat used for e.g. DHW and space heating. Collector efficiency depends on optical properties and heat loss.

Concentrated solar power: Sunlight is concentrated by mirrors or lenses to a small absorber area and converted to heat. A heat engine converts the heat to electric power.