options for energy_sustainability

Options for energy sustainability

The outline of the problem

• Human activities often require energy– Power for equipment– Power for lighting– Heating for thermal comfort– Hot water for hygiene– Cooling in hot weather

• All require external supplies of energy• Majority of external energy comes from

non-renewable sources - fossil fuels

What are fossil fuels?

• All “fossil” fuels are a product of ancient biological activity

• Living plants capture and fix carbon dioxide (CO2) and water (H2O) as hydrocarbons through photosynthesis– Solar energy is converted to chemical energy,

stored in the hydrocarbons

• All life depends on this process and it also releases oxygen (O2)

• Ancient plant and animal remains have been preserved, in some cases, as fossilised hydrocarbons – fossil fuels

Oil and gas: fluid fossil fuels

Coal and peat: solid fossil fuels

What are the problems with using fossil fuels?

• They are finite resources which will eventually run out - sustainability?– There are vast reserves of coal but other

fossil fuels are beginning to run out already• Burning fossil fuels (combining a

hydrocarbon with oxygen) to release the stored chemical energy as heat reverses the photosynthetic process:

CH4 + 2O2 >> CO2 + 2H2O

Heat energy released

Input: methane and oxygen

Output: carbon dioxide, water and heat

Why is carbon dioxide (CO2) a problem?

• Carbon dioxide (CO2) is essential for life and maintenance of life supporting temperatures, but…

• An increase in atmospheric carbon dioxide could lead to significant global temperature rises through the “greenhouse effect” which will disrupt human societies– Rising sea levels - flooding– Changes in agricultural systems– Changes in water supply

What is the greenhouse effect?

CO2


CO2 in atmosphere does not let long wave infrared (IR) radiation through

LWIR

CO2



LWIR

SWIRCO2



The Earth, warmed by short wave IR, emits long wave IR which cannot escape through the CO2 in the atmosphere. There is a one way traffic in energy input temperatures rise

LWIR

SWIRCO2

Reducing CO2 emissions aka low carbon technologies

• Use renewable, non-finite energy sources:– Direct solar radiation – Geothermal energy– Hydro electric power– Tidal power– Wind– Biogas– Nuclear energy

• Design buildings to make best use of environmental energy

Designing to use the Sun

• Solar energy can be exploited three ways– Photo-voltaic electricity generation– Water heating– Direct solar heating of the building fabric

• The last two make direct use of the greenhouse effect themselves

Photo-voltaic electricity generation

• Light photons fall on “photo-cells”, displacing electrons and thus generating a current

• Efficiency low and cost high

• Pay back periods only economic due to government sponsored “feed in tariffs”

• They don’t work in the dark

• Energy very difficult to store

Solar Water heating

• Black painted water pipes under glass are heated by direct solar radiation using the greenhouse effect

• Pay back still very long in UK climate but should become standard fitting on all new houses.

Direct solar heating of the building fabric

• Exploit solar energy to the maximum– Consider building orientation– Exploit the greenhouse effect for building

heating– Control excessive solar heating through

shading– Store day time solar heat in mass

construction for night time use

Orientation: the sun is in the south…

• Position building and locate windows to maximise solar gain.

• Build heavy masonry walls to store the heat to re-emit during the night

Orientation: the sun is high in summer, low in winter

The Brise soleil

Geothermal energy for individual buildings

• High temperature geothermal energy– Dependent on hot rocks close to the

surface– Not viable in the UK but used intensively in

Iceland• Low temperature “ground source” heat

– Exploits the temperature differences between the atmosphere and the sub-surface ground

– Concentrates low quality heat using “heat pump” technologies

Ground source heat

• UK air temperature varies between -10 and +30ºC

• 1.5 metres below ground the temperature is constant around 10ºC

• This temperature difference can be exploited for heating or cooling.

1.5m

10ºC constant temp

-10ºC>+30ºC

Simplest ground source heating

• Build underground; earth sheltered houses

Technical exploitation of ground source heat

• “Pump” the heat to the surface

• To be effective, air-conditioning technology must be exploited, in reverse.

• “Heat pumps” concentrate low grade heat to get higher temperatures

1.5m

10ºC constant temp

Large coiled pipe full of refrigerant fluid in the ground

Pump and valve form the “heat pump” which concentrates the heat

Small coiled pipe full of refrigerant fluid in the building

Wind energy

• Wind generation of electricity is renewable and non-polluting (when generating) but…

• It is intermittent– Where do you get

power from when it is not running?

• It is highly dependent on high wind speeds

Is there enough wind energy to be practically useful?

• The theoretical energy output is proportional to the wind speed cubed– If the wind speed doubles, the theoretical

energy output goes up eight times (2x2x2)– If the wind speed halves, the theoretical

output falls to an eighth (2/2/2)• High steady wind speeds are needed

for useful energy generation• Domestic scale installations in lowland

areas simply cannot deliver useful quantities of energy.

Biogas generated from waste

• Fermenting organic waste generates methane, a main constituent of natural gas.

• Digesting farm waste generates gas and solid residue is a good fertiliser

• Food waste can be digested in urban locations to generate biogas and reduce quantity of waste

• Gas can be burned to generate electricity or fed directly into the gas supply network

Summary

• Solar energy can be exploited for heat/hot water, but electricity generation is still in its infancy.

• Wind power is ineffective at domestic scale

• Significant quantities of environmental energy can be harvested through good building design

• Biogas generation can potentially provide fuel or energy, but needs to be fairly large scale to be significant.