Cleaning up on renewables
This is the third article in a series of four about environmental sustainability.
Once resource use has been minimised through conservation and efficiency the next key strand for achieving sustainability is to use renewable and non-polluting resources – especially to replace fossil fuel use.
Renewable energy utilises resources which are naturally replenished. Renewable energy technologies range from solar, wind and water power for generating electricity to biomass for heating and biofuels for transportation.
On a global basis 13% of primary energy comes from renewable sorurces. In New Zealand this figure is nearly 30% due to significant hydro and geothermal resources we utilise.
An important feature is that renewable energy sources and technologies are suited to small-scale applications whereas fossil fuel sources are not (unless you have a small coal seam in your back yard!).
The cost of renewable sources of energy varies. Geothermal power is quite cheap whilst photo-voltaic generation is still very expensive. As new technologies mature their relative costs will tend to decrease.
Over 70% of New Zealand’s total primary energy supply is fossil fuels – used mostly for transport and heating. Most of our transport fuel comes from overseas. As fossil fuel resources are depleted and prices go up they will need to be replaced by alternative, renewable sources. The cost of carbon emissions will also be added to fossil fuel prices which will make renewables more competitive and ultimately cheaper.
The cost of energy is going to continue to increase. It is now abundantly clear that energy should never have been as cheap as it was in the past and we must now pay the price for the damage done.
The majority of renewable energy technologies are directly or indirectly powered by the sun. The sun’s energy causes winds and ocean currents, it causes precipitation tapped by hydroelectric projects and through the process of photosynthesis the sun energises the growth of plants used to create biofuels.
The other main source of renewable energy is geothermal power utilising the earth’s heat.
Airflow runs wind turbines. Areas where winds are stronger and more constant, such as offshore and high altitude sites, are preferred locations. But e ven in the best areas wind is not constant (wind cannot be stored like water in a dam) and must be used in conjunction with other forms of generation.
Wind power is the fastest growing of the renewable energy technologies, though it currently provides less than 0.5% of global energy. However, globally, the long-term technical potential of wind energy is believed to be five times total current global primary energy production.
In New Zealand the amount of wind energy installed was about 170 MW in 2005 which is less than 2% of the total electricity generated and less than 0.3% of total primary energy. However there is potential in New Zealand for wind to generate more than 2,500MW – enough electricity for over one million homes.
Energy in water (in the form of motive energy or temperature differences) can be harnessed and used. Since water is about 800 times denser than air even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy.
There are many forms of water energy, such as large-scale and micro- hydro systems, tidal power and w ave power
The radiant energy in sunlight can be utilised in different ways. Electricity can be generated using photovoltaic solar cells or concentrated solar power. Sunlight is also commonly used to heat water using solar-thermal panels and of course the sun can heat houses and buildings directly through passive solar design.
The International Energy Agency classifies geothermal power as renewable whilst others believe that it is not. Geothermal energy is energy obtained by tapping the heat of the earth itself. It is expensive to build a power station but operating costs are low resulting in low energy costs.
In New Zealand geothermal energy is used directly for heating and also for electricity production.
New Zealand currently has a geothermal generating capacity of 435MW which produces about 7% of total electricity supply. It is estimated that the potential electricity generating capacity of our geothermal resources is 2600MW which is about 75% of the country’s peak electricity demand.
Plants use photosynthesis to grow and produce biomass. Biomass can be used directly as fuel or to produce liquid and gas biofuels.
All biofuels, being carbon-based, create C02 emissions when used. However if the biomass is regrown the C02 emitted gets reabsorbed from the atmosphere and there is little no net effect.
Solid biofuels are used directly usually in the form of combustible solids – wood, solid municipal biogenic waste, combustible field crops and agricultural waste. Most sorts of biomass can be burnt to heat water and to drive turbines. Solid biomass can also be gasified and used as below.
Liquid biofuel is usually either a bioalcohol such as ethanol or a bio-oil such as biodiesel. Biodiesel can be used in modern diesel vehicles with little or no modification to the engine and can be made from vegetable and animal oils and fats (both recycled and new). Biodiesel creates 20-40% less emissions than petrol.
In New Zealand ethanol and biodiesel are are being phased in as blends with petrol and diesel.
Biogas can easily be produced from current waste streams, such as: paper production, landfills, sewage and cattle effluent (good news for the dairy industry) These wastes are slurried and allowed to naturally ferment, producing methane gas which can be burnt or converted into higher grades of gas.
The remaining waste after biogas extraction is often more suitable as fertiliser than the original biomass.