Renewable energy: general aspects

Juan Manuel Martín Ordax

Main concepts

Renewable energy is energy obtained from naturally repetitive and persistent flowsof energy occurring in the local environment.

Non‐renewable energy is energy obtained from static stores of energy that remainunderground unless released by human interaction.

It is necessary to:

• Consider the resources for energy in the environment• Existing technologies to harness the resources• Make an efficient use of energy• Consider the impact of the use of technologies• Evaluate the cost‐effectiveness of the energy supply

Quote from the IPCC‐Intergovernmental Panel on Climate Change

For a renewable energy resource to be sustainable, it must be inexhaustible and not damage the delivery of environmental goods and services including the climate system. For example, to be sustainable, biofuel production should not increase netCO2 emissions, should not adversely affect food security, nor require excessive use of water and chemicals, nor threaten biodiversity. To be sustainable, energy must also be economically affordable over the long term; it must meet societal needs and be compatible with social norms now and in the future. Indeed, as use of RE technologies accelerates, a balance will have to be struck among the several dimensions of sustainable development. It is important to assess the entire lifecycle of each energy source to ensure that all of the dimensions of sustainability are met.

IPCC, 2011

Energy sources

The main energy sources are:

• Sun• Movement and gravitational potential (Earth, Moon and Sun) • Geothermal energy• Nuclear reactions• Chemical reactions

Renewable energy supplies: Wind, solar, biomass, tidal. Conventional energy supplies: Coal, oil, gas, radioactive ore.

Sources:

• Renewable: Natural local environment.• Conventional energy: Concentrated stock

Energy sources

Contrast between renewable and finite energy supplies.

(Source: Renewable Energy Resources, 3rd edt) 

Energy sources: general comparison

(Source: Renewable Energy Resources, 3rd edt) 

Energy sources

The main energy sources are:

• Sun.• Movement and gravitational potential (Earth, Moon and Sun) • Geothermal energy• Nuclear reactions• Chemical reactions

Renewable energy supplies: Wind, solar, biomass, tidal. Conventional energy supplies: Coal, oil, gas, radioactive ore.

Sources:

• Renewable: Natural local environment.• Conventional energy: Concentrated stock

Environmental energyNatural energy currents on the Earth, showing renewable energy systems. Note thegreat range of energy flux (1:105) and the dominance of solar radiation and heat.Units are Terawatts (1012W).

(Source: Renewable Energy Resources, 3rd edt) 

Renewable sources: features

Intensity and frequency properties of renewable sources

(Source: Renewable Energy Resources, 3rd edt) 

Supply and demand

(Source: Renewable Energy Resources, 3rd edt) 

a Maximum energy flow for minimum sizeof device or system requires low resistanceto flow at D, E, and F

b Negative feedback control for a systemwith finite sources allows fuel to be savedas load decreases.

c Negative feedback for a system withpurely renewable input spills energy beyondthat required by the load.

d Positive feedforward load managementcontrol of the supply, separating ‘high‐priority’ and ‘low‐priority’ loads, so thattotal load at E may be matched to theavailable supply at D at all times.

Supply and demand

(Source: Renewable Energy Resources, 3rd edt) 

e Energy storage allows the dynamiccharacteristics of end‐use to be decoupledfrom the supply characteristics.

f A large grid system can incorporate bothfeedback (to adjust the supply to thedemand) and feedforward (to switch on‘low‐priority’ loads only when the supply isadequate).

Technical efficiency of an energy device

It usually means the useful energy supplied as a fraction of the input energy.The best efficiencies, according to this definition, of various common devices are:

• Power stations (electricity to grid/heat input; with no use of rejected heat): coal andoil ~35%, gas turbine ~45%, nuclear ~30%.

• Cars (motive energy/heat from combusted fuel) ~10%.

• Cyclist while racing (rate of motive energy/rate of food metabolism) ~7%.

• Regular cyclist over a year (annual motive energy/annual food metabolism) <1%.

• Electricity generator (electricity out/shaft power in) ~95%.

• Bicycle (motive power out/pedal power in) ~85%.

• Incandescent electric light3 (visible light out/electricity in) ~2.5%.

• Light emitting diode (LED) (visible light out/electricity in) ~12%.

Technical efficiency of an energy device

The main factors to be calculated are:

Technical efficiency of an energy device

Some examples of values for these factors: (Z = Annual capacity factor; TF= Full loadhours)

• Hydroelectricity, continuous water, 1% maintenance downtime Z~99% (TF ~ 8700 h/y)

• Hydroelectricity, Scotland, with 30% water availability Z~30% (TF ~ 2600 h/y)

• Wind turbine in central Germany (moderate winds) Z~ 18% (TF ~ 1600 h/y)

• Wind turbine in Wellington, New Zealand (‘the windy city’) Z~ 45% (TF ~ 3900 h/y)

• PV tracking solar panel in northern Chile (nearly cloudless) Z~ 40% (TF ~ 3500 h/y)

• PV fixed orientation solar panel, central England (often cloudy) Z~ 10% (TF ~ 880 h/y)

• Biomass combustion for thermal power plant Z ~ 90% (TF ~ 7900 h/y)

• Tidal range (barrage) power Z ~ 25% (TF ~ 2200 h/y)

• Wave power, vigorous site (potential)  Z ~ 30% (TF ~ 2600 h/y)

• Tidal stream (current) power (potential)  Z ~ 20% (TF ~ 1800 h/y)

• Ocean thermal power (OTEC potential) Z ~ 90% (TF ~ 7900 h/y)

(Source: Renewable Energy Resources, 3rd edt) 

Factors influencing capacity factors

Using energy efficientlyIndicative diagram of energy flows from source to end‐use service. 

(Source: Renewable Energy Resources, 3rd edt) 

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