Issue 59: Renewable Energy

Issue 59
July/August – 2001
Story Title: Renewable ENERGY… Resources that don’t cost the Earth!
Author: Steven Carruthers

Wind power continues to dominate the world’s renewable energy development. World Market Update 1999, the fifth such report authored by Danish company BTM Consult, reports that 51% more capacity was installed in 1999 than in 1998 (measured in megawatts). New installed capacity was 3,922 MW, which brings the total wind power capacity to 13,932 MW worldwide, a 37% increase over 1998. Annual average growth from 1995-1999 was 40%. The report predicts wind power capacity will almost treble by 2005 and then grow again by a factor of two-and-a-half by 2010. With recent developments in offshore wind farms, most growth is expected to come from Europe.

The rising cost of fossil fuels, energy shortages, and a growing concern about global warming, have led to major advances in renewable energy systems, large and small.

Like the energy crisis of the early 1970’s, the spiraling costs of fossil fuels and energy shortages in the “noughties” have accelerated research and development into renewable energy. It’s also being fast-tracked by a growing concern about global warming. The combustion of fossil energy from sources such as coal, oil and gas, releases large amounts of carbon dioxide and other air pollutants, leading to detrimental effects such as acid rain, respiratory diseases, as well as contributing to the global greenhouse effect.

An alarming new study predicts that Australia will be one of the regions worst hit by climate change. Commonwealth Scientific and Industrial Research Organisation (CSIRO) scientists predict that temperatures across Australia will increase by up to 6°C by 2070. They predict that in areas that experience little change or an increase in average rainfall, more frequent downpours are likely. Conversely, there will be more dry spells in regions where average rainfall decreases.

“We may also see more intense tropical cyclones, leading to an increase in the number of severe oceanic storm surges. Rises in sea level would aggravate this effect,” said Dr Whetton from CSIRO Atmospheric Research.

The study reports that the sea level is likely to rise at a rate of between 0.8 and 8cm per decade, reaching 9 to 88cm above the 1990 level by the year 2100.

CSIRO scientists collaborate with the Intergovernmental Panel on Climate Change (IPCC), which is the international group charged with assessing the latest science on the greenhouse effect. CSIRO’s new projections incorporate IPCC findings. In its last CSIRO climate projections, five years ago, the temperature range estimated for 2070 was 0.6°C to 3.8°C. The current forecasts for warming are from 1°C to 6°C. According to Dr Peter Whetton: “Rising concentrations of greenhouse gases are the culprit”.

Global warming is expected to have a significant impact on agricultural production. Higher carbon dioxide concentrations will increase plant productivity and the efficiency with which plants use water. A moderate rise in temperature will increase plant growth in temperate areas, but may reduce it in other regions. Warmer conditions will reduce frost damage in many crops, however, some plants need cold weather to set fruit, so some yields may decline. According to the study, a rainfall decline of 20% with temperature increases of more than 1°C, will lower yields for many plants, including important cereal crops. Global warming will also result in the spread of many pests and diseases. Tropical pests may spread to warmer regions, and temperate pests may move to cooler areas.

“The net effect on agriculture will be a trade-off between the positive impact of higher carbon dioxide and the negative effect of lower rainfall and higher temperatures,” said Dr Whetton.

The CSIRO report has renewed calls for the Federal Government to support the Kyoto Greenhouse Protocol, which it says it favours but won’t ratify unless America does; the U.S. Government says it is convinced that new technology – not reducing energy consumption – is the answer to climate change, and has abandoned the treaty altogether. The Federal Environment Minister, Senator Robert Hill, says that despite the uncertainty surrounding the Kyoto Protocol, Australia will push ahead in its efforts to combat global warming.

“We are already taking significant steps to reach our Kyoto target with $1 billion already targeted through specific programs to reduce greenhouse emissions,” he said.

Australia contributes 1.4% of the world’s greenhouse gas emissions, with the energy sector accounting for almost 80% of that in 1999, according to the latest available data. As part of its climate change package to reduce the country’s reliance on fossil fuels, the government has committed almost $400 million to initiatives to fast-track a viable, renewable energy industry. Some of these initiatives include funding assistance in the development of Australia’s largest solar farm. Using world-first solar technology, the Broken Hill Solar Power Station covers 20 hectares and produces one megawatt of power. The new technology developed by Solar Systems Pty Ltd comprises a solar dish concentrator that magnifies the intensity of the sun by 500 times.

With Australia’s demand for electricity set to increase by 35% between now and 2012, the government has passed legislation that requires energy producers to source an additional 2% of their power from renewable energy sources. It predicts that this will generate $2 billion worth of new investment in the renewable energy industry.

In April 2001, the government introduced Renewable Energy Certificates, designed to ensure that 9,500 GWh of additional renewable energy is generated each year by 2010 – enough energy to supply the electricity needs of a city of four million people. The tradable certificate is a form of currency used to demonstrate compliance with the government’s renewable energy targets.

At the micro level, the rising cost of fossil fuels and ongoing energy shortages in consumer societies, have seen major advances in renewable energy systems for stand-alone dwellings, residential districts, schools and hospitals, as well as for greenhouses. Because of the need to provide energy storage or a backup system, some of these energy systems are occasionally more expensive to purchase than their conventional fuel counterparts. Even though systems may cost less to operate over their lifetime, some purchasers only see the initial cost difference.

What is renewable energy?
Renewable energy is the generation of electricity, transport fuel, process heat and other end-use forms of energy from primary sources that are not depleted by such generation. Examples of renewable energy resources include:

Solar thermal
Solar photovoltaic
Hydroelectric
Wind
Biomass
Geothermal
Wave & Tidal

Solar Energy
The sun has been the primary energy source for life on earth for around 4.6 billion years. Its power reaches us as light or electromagnetic waves and continues to create most of the forms of energy we use today. If examined in the broadest sense, the sun creates the air temperature differences which provide the air currents that make wind energy possible; it provides the light to grow the biomass fuels, such as wood and grain used to distil ethanol; it provides the moving force behind the Earth’s water cycle, thus making hydroelectricity possible; even the fossil fuels began as vegetation long ago in the Earth’s history.

Free energy
One reason for the popularity of solar energy is the perception that it is free. Perhaps a better choice of words would be to say that it is unmetered and renewable. There are, as with any energy source, costs involved in the equipment used to collect, store and distribute the energy.

There is also the need to have clear access to the sun during peak solar hours. For instance, it would not make sense to locate an installation in forested areas or in the shadow of a building because it would not have adequate solar access. Furthermore, the amount of solar energy available varies according to the time of day, the time of the year, the whims of the weather and the region of the country.

Types of solar energy
In general, solar energy systems can be categorised as being one of two types: Thermal Systems, which use the sun’s energy in the form of heat, and Light-Utilising Systems, which use the sunlight directly to provide energy or lighting.

Active thermal systems
Active thermal systems are used to provide heat for thermal comfort in buildings (space heating) and water heating. Residential water heating is the most common application for active systems, but it is also effective for heating larger volumes of water for commercial purposes.

Active systems use mechanical equipment such as pumps and fans to regulate and distribute the energy collected from the sun. A typical system consists of one or more flat plate collectors connected to a storage and distribution system. The flat plate collector is essentially a well insulated box with a dark metal absorber plate underneath a transparent cover. A heat transfer fluid, either air or a liquid, is moved through the collector, where it picks up heat from the absorber plate. The fluid is then directed to a storage area (typically a rock bin for air systems, a water tank or some type of phase change material for liquid systems) where it is available for use in the space or water heating system.

Figure 1 – Flat-plate solar collector

For space heating, either an air or liquid system may be used. In air systems, the heated air from storage is used to heat the house; in liquid systems, a heat exchanger is used to transfer heat from the liquid in storage to the air to be distributed through the building.

Water heating systems typically use liquid collectors. The collector fluid may either be water, or a different fluid with less freezing potential. In this case, a heat exchanger is used to transfer the heat from the collector fluid to the water.

Passive thermal systems
Like active systems, passive solar systems are used to provide space and water heating for buildings. Unlike active systems, they do not use pumps or fans to store or distribute heat. Instead, they rely on the natural heat transfer forces of conduction, convection and radiation to distribute the heat collected.

Solar air heater

In the northern hemisphere, passive space heating systems consist of south-facing glass to collect heat (north-facing glass in the southern hemisphere), and massive building materials within the structure (such as brick, concrete, stucco, tile, or containers of water) to store the heat. These massive materials have the ability to absorb heat, and then release it slowly to the surrounding, cooler areas.

Photovoltaic systems
Photovoltaics is the process of converting sunlight into electricity by means of a photovoltaic cell. The photovoltaic cell is a solid-state device composed of thin layers of semiconductor materials which produce an electric current when exposed to light. Single cells are connected in groups to form a module, and modules are grouped to form an array. The voltage and the current output from the array depend upon how the system is configured.

Photovoltaic cells produce direct current (DC) electricity, the type of electricity contained in batteries. However, most electrical equipment is designed to use alternating current (AC) electricity, the type available from a standard wall socket. When AC current is required, an inverter is added to the photovoltaic system to change the current from DC to AC, but this will incur a 10-15 percent loss of power output.

Photovoltaic-generated electricity has many applications. It has already become a permanent fixture in the consumer products market by providing energy for products with small power requirements, such as solar calculators and watches. Other applications include water pumping, navigational signals, lighting, electric fence charging, vehicle battery charging, radio relay stations, and utility-scale electricity generation. The latter, while feasible, is not commonplace due to the current low costs of producing electricity from coal or nuclear energy.

Photovoltaic systems are often used in remote locations, away from an electricity grid. However, these systems are not commonplace, owing to the high cost of photovoltaic arrays compared to the low cost of diesel fuel needed to run a power generator.

Solar ponds
Solar ponds are used in countries such as Australia and Israel, where there is an abundance of strong sunlight. The solar pond consists of a body of water used to collect and store solar energy. The pond, natural or man-made, contains salt water, which acts in a different way to fresh water. In a fresh water pond, for example, water heated by sunlight would rise to the top by natural convection, while heavier cool water would sink to the bottom. However, salt water is heavier than fresh water and will not rise or mix by natural convection, thus creating a temperature inversion layer within the pond. Fresh water forms a thin, insulating surface layer at the top, while the salt water underneath becomes hotter with depth. Temperatures over 90°C (200°F) are not uncommon.

The main use for solar ponds lies in electricity generation. Heated brine is drawn from the bottom of the pond and piped into a heat exchanger where heat converts liquid refrigerant into a pressurised vapour, which in turn, spins a turbine, generating electricity.

Daylighting
Daylighting is the use of natural light to illuminate structures during the daylight hours. It’s not a measurable source of energy; rather, it’s a method of displacing energy which otherwise would be used to provide light for buildings. Since lighting represents a major cost in a building’s overall energy use, daylighting will reduce the building’s energy consumption, as well as provide a healthy environment for the occupants.

A solar house where photovoltaic cells are used to heat water and generate electricity. Daylighting is also used to warm interior spaces.

Daylighting is more of a design issue than a technology issue, although technology has helped to make it a more feasible option by providing more advanced glazing materials.

Micro-Hydroelectric Energy
For thousands of years, humans have utilised the power of falling water to turn water wheels for the grinding of flour. However, it has only been in the past 100 years or so, that we have discovered how to use this renewable energy source for the creation of electrical power.

The driving force behind hydropower is the ‘hydrologic cycle’, as illustrated in Figure 2. In this cycle, the sun’s energy is absorbed by surface water and soil at low elevations, causing water to evaporate, forming water vapour. Water vapour is also produced from transpiration of plants. Clouds form from this water vapour and this subsequently condenses to form water droplets. These water droplets eventually fall as precipitation.

Figure 2 – The hydrologic cycle

If this precipitation lands on areas of high elevation, the water will have relatively high potential energy. As gravity causes this water to run off in streams, its potential energy is converted into kinetic energy, frictional heating, and mechanical energy. Hydropower technologies attempt to convert some of this energy into mechanical or electrical energy.

Hydroelectric systems convert the energy embodied in flowing water into electrical energy. Micro-hydroelectric systems are those that generate less than 20 megawatts of electricity. These are small-scale systems with little negative environmental impact and can be implemented by private individuals and small companies. Pumped storage allows excess energy to be stored for later use. Run-of-the-river plants derive energy from a water flow without disrupting it as much as conventional hydroelectric power plants. Unlike conventional hydroelectric, micro-hydroelectric systems use no dams or very small dams that impound little water. Because only small diversion weirs or intakes are necessary for micro-hydroelectric systems, they do not typically interfere with fish migration or threaten stream ecology.

Power in water
The power available from a micro-hydroelectric system depends on the available elevation change, the stream or river flow rate, and the overall micro-hydroelectric system efficiency. Overall system efficiencies of 30% to 60% are common in micro-hydroelectric systems.

System components
Figure 3 is a diagram of a typical micro-hydroelectric plant with a generating capacity of the order of 100 kW. Smaller micro-hydroelectric plants may not include all of the components shown in this figure.

Water flows down the feeder canal from the intake to the forebay. Usually, the canal is made of earth-stone or concrete and is fitted with a trash rake and screen to keep out floating debris and marine animals. The feeder canal dumps into the forebay. This is a tank that holds water between the feeder canal and the penstock. The function of the forebay is to ensure that the entrance to the penstock is fully submerged so that the designed head is available and air is excluded from the turbine.

The penstock is the pipe connecting the forebay to the turbine. This is the high pressure pipe in the system. The head available to the turbine is developed by the change in elevation between the inlet of the penstock and the exit of the turbine. The turbine is the device that converts the power available in water into mechanical energy. A generator connected to the turbine converts the mechanical energy into electric energy. As the water exits the turbine, it is re-introduced to the stream via the tailrace.

In smaller systems, the dam and fish ladder may be replaced by a small diversion weir. The feeder canal and forebay also, may be much less defined or may not be present at all.

Many turbine types are available for micro-hydroelectric systems. Pelton turbines and Turgo turbines are the most common types of micro-hydroelectric turbines available in the United States. Other types are the cross flow turbine, the Francis turbine, the propeller turbine, and the traditional water wheel. The water wheel is the oldest of the turbines and is not commonly used these days.

System types
Micro-hydroelectric plants can produce alternating current (AC) or direct current (DC) electric power. AC micro-hydroelectric plants are usually interconnected with the utility grid but can be stand-alone systems if ballast loads are used. AC plants require systems to control the voltage and frequency of the power being generated. These plants are usually tens to hundreds of kWs in size. Smaller hydroelectric turbine generator sets usually produce DC power that is used to charge large battery banks. These systems can be used to run DC loads, or a DC to AC inverter can be employed, allowing the operation of typical AC loads. These systems may or may not be interconnected with the utility.

Figure 3 – Diagram of a typical 100kW micro-hydroelectric plant.

Cost
Micro-hydroelectric systems can be installed for US $2,000 to $5,000 per peak kW capacity. Variations in site conditions and water resources account for the wide variance in price. Low head applications are more expensive than high head applications. Low head availability requires high flow rate to reach desired power output. High flow rates require larger and more expensive equipment. High head, low flow systems are the most economical.

Equipment and service availability
Residential-sized micro-hydro turbines with a capacity up to 1.5kW are readily available from renewable energy catalogues and can be installed by do-it-yourselfers. Larger systems would need to be designed and purchased from a micro-hydroelectric specialty firm and installed by a contractor. (Firms that can provide this service can be found in Hydro Review journal published by HCI Publications Inc., Kansas City, Missouri; pH: +1 [816] 931-1311.)

Wind Power
Wind is a form of solar energy. This is because it is the heat from the sun that ultimately forms the earth’s wind patterns. On a micro level, wind is generated by the movement of air from regions of high pressure to regions of low pressure, with these differential pressures established by thermal gradients. While wind directions may seem to change with frequency, in most places there are prevailing wind patterns that make it easier to predict and harness the power of wind.

Wind technology overview
Electric generation has recently become the most common use for wind energy. This is effected by the use of spinning blades to turn the armature in a generator located directly behind them at the top of the tower. The energy from this generator is then rectified before being converted to AC electric output through an inverter.

The power available from the wind is a function of the cube of the wind speed, which means that, all other things being equal, a turbine at a site with 10 miles per hour (mph) winds, will produce twice as much power as a turbine at a location where the wind averages 8mph.

In general, winds exceeding 11mph are required for cost-effective applications of small grid-connected wind machines while windfarms require wind speeds of 13mph. For applications that are not grid-connected, of course, these requirements may vary, depending on the other power alternatives available and their costs.

As technology has matured, available turbine sizes have grown. Turbines installed today for utility grid power are typically 250kW to 900kW with some machines available that can produce over 1 MW. These are massive machines that have blade spans of over 150 feet, resting on towers over 200 feet tall. A single 750kW turbine can power over 50 homes. Wind farms (clusters of wind turbines) may have as many as 500 turbines producing over 400MW of power.

And just as wind turbines are getting bigger, they also are getting smaller, with machines down to 200W commercially available for residential, recreational and general off-grid use. Utility grid power is only one of many applications. Small wind turbines are also used on sailboats to charge batteries, in homes to offset electricity use, to power research stations in the Arctic and Antarctic, and by farmers to produce electricity for well pumps.

There are two general classifications for wind turbines: the horizontal axis (HAWT) and the vertical axis (VAWT). Although horizontal axis machines are more common, the vertical axis machines have the advantage of not being sensitive to wind direction.

Sophisticated controls technology enables these systems to be optimised for nearly all conditions, although large wind farms are usually located along a ridgeline, mountain pass or the coast where strong winds come consistently and from the same direction. Typically, these big machines are designed and controlled so that their output is AC current, which eliminates the need for a DC-to-AC inverter.

Interested in investing in a wind system?
Like solar, wind energy systems usually have a high up-front cost with little or nil operating cost for the life of the system. The viability of a wind system will depend on three issues:

1. Available wind at the site to meet your needs;
2. Regulatory barriers that may prohibit the installation of a wind system; and
3. Whether the initial investment can be returned in a reasonable amount of time.

Biogas Energy
The creation of biogas is not new to our planet. Since the beginning of time, it has been part of Earth’s natural processes. Biogas, which mainly consists of methane and carbon dioxide, is the primary waste product of anaerobic bacteria as they digest organic material. The process occurs in the absence of oxygen. These conditions occur naturally in swamps, marshes, bogs, deep bodies of water, and in the digestive systems of many animals. Although biogas has been around for millions of years, it wasn’t used by humans until 1895 in England, when it was used to fuel street lamps.

Since the late 1800’s, anaerobic digestion has been used to treat waste, generating excellent soil amendments, reducing greenhouse gases, diminishing odour problems, and providing a renewable energy source. Today, anaerobic digestion is used to treat city sewage, hog, chicken and cow waste; and municipal solid waste where the methane gas is used for heating, electricity and steam generation, as well as to supply fuel for natural gas lines. The by-product, termed effluent, is rich with nutrients and is often sold as a soil amendment for nurseries or farms. Biogas systems are also excellent for controlling odours and reducing the release of ozone-depleting methane into the atmosphere. In fact, the first farm-based biogas system in the U.S. was built for odour control at a hog farm in Iowa. The most notable biogas systems in Australia can be found at Lucas Heights. The Narre Warren biomass power station in Victoria sits over a land fill tip and converts methane gas into enough electricity to power 11,000 home, a paper recycling plant, and Mayas Roses, a hydroponic rose farm operating eight greenhouses.

Anaerobic process
The anaerobic process takes place in an enclosed tank or deep water catchment, usually called a digester. The digester prevents waste from being exposed to oxygen. The process takes place in three anaerobic stages: fermentation; hydrogen, carbon dioxide and acetic production; and formation of methane. During the fermentation stage, fermentative bacteria break down organic material into fatty acids, alcohol, carbon dioxide, hydrogen, ammonia and sulphides. During the second stage, acetogenic bacteria break the material further down to hydrogen, carbon dioxide and acetic acid. In the final stage, methanogenic bacteria form methane, which can then be used as a fuel source.

The anaerobic process is greatly affected by temperature. Most systems operate in either the mesophilic range (68-113°F) or the thermophilic range (113-150°F). The thermophilic range allows for the largest loading rate of a digester as well as increasing the destruction of harmful pathogens. Although little research has been done at lower temperatures, it has been observed that digestions can also take place in the psychrophilic range of less than 68°F. Higher temperature systems generally have a greater rate of waste processing per unit volume of digester but tend to be more complicated to operate, and require more equipment. Many of the recently installed systems in North Carolina are of the psychrophilic and mesophilic type.

The biogas generated from either of the three digester types and operating temperatures consist of primarily methane (60% to 80%, about 600-800 BTU/ft3), and carbon dioxide (20% to 40%). There are normally traces of corrosive hydrogen sulphide, which must be scrubbed out before utilising the biogas as a fuel source.

System components and digester types
A biogas system normally consists of a waste collection system, waste pre-processing system, digester, and some final effluent handling and storage. If electric power is generated directly from the biogas, a gas scrubber and generator are also part of the system.

A waste collection system varies dramatically and is dependent upon the type of waste stream and waste handling practices. Before entering a digester, waste may need to be mechanically shredded so as to improve system performance. Often, the waste stream is also preheated before entering a digester.

The three types of digesters most commonly used today are: covered lagoons, complete mix digesters, and plug flow digesters. Covered lagoons are used when the waste stream is less than 2% solids. The waste is stored in a deep lagoon and covered with a floating cover to trap escaping biogas. Complete mix digesters have engineered tanks, located above or below ground, that treat waste in the range of 3% to 10% solid content. These types of digesters generally require less land and have a shorter treatment time than covered lagoons. Plug flow digesters are engineered, heated rectangular tanks that treat solid waste in the range of 11% to 13% solid content.

Biogas also can be generated by the decomposition of municipal solid waste. Currently, there is ongoing research to develop digesters that will break down the organic portion of a waste stream to generate biogas while reducing the amount of garbage that must be landfilled. For the waste already in the landfill, biogas can be recovered and utilised. This is currently happening at over 100 landfills throughout the United States.

Clean Air Calculator
The average household can save about seven tonnes of CO2, being released into the atmosphere each year, by switching to renewable energy – the same effect as taking two cars off the road for a year. To see how much greenhouse gas you could have saved, visit the EnergyAustralia website where you will find the ‘Clean Air Calculator’. Simply enter your quarterly bill account to receive an automatic response. The Clean Air Calculator can be found at:

Website:
http://www.energy.com.au

Geothermal Energy
A vast amount of energy is stored and produced in the upper 10 kilometres of the Earth’s crust. If only 1% of this energy could be economically tapped, it would be 500 times greater than all the known oil and gas resources. This renewable energy resource is called geothermal energy, and is currently used in more than 40 countries.

Figure 4 – Ground source vertical well ground connection.

Geothermal energy under the Earth’s surface comes from hot rocks and liquids produced by recent volcanic activity. One of the principal sources of geothermal energy comes from large reservoirs of hot water trapped in the permeable rock formations below the surface. These reservoirs are called hydrothermal reservoirs. This energy can be recovered and put to use.

Figure 5 – Ground source heat pump: horizontal trench ground connection.

Thermal capacity is another type of geothermal energy. The ground just below the surface at ambient temperatures can serve as a heat reservoir. The stable ground temperature just below the surface can be used to heat and cool residential and commercial structures with geothermal heat pumps.

Geothermal heat pump systems
Electrical energy use can be reduced 30% to 60% by replacing standard electrical heating and cooling systems with geothermal (ground source) heat pumps. These energy savings will also reduce the negative environmental impacts occurring with fossil fuel emissions.

Ground source heat pumps can be used for residential and commercial heating and cooling, and for domestic hot water. The ground just below the surface stays about the same temperature year round. The underground temperature is greater than the outside air temperature in the winter months, and less than the outside air temperature in the summer months. The ground source heat pump can be used to draw the geothermal heat from the ground in the winter and release the heat back into the ground in the summer.

The three principal components of a ground source heat pump system are: the heat pump, the ground connection, and the conditioned air distribution system. The heat pump component is the same as a standard heat pump that uses outside air as a heat sink/source, except it is connected to a ground loop and uses the ground, rather than the air, as a heat sink/source. The ground connection or ground loop is buried underground adjacent to the facility to be heated and cooled.

Figure 6 – Ground source heat pump pond connection.

The ground loop can be buried in various configurations. The loops of plastic pipe can be placed vertically in a well, several hundred feet deep, and back-filled to provide good ground contact (see Figure 4). The loop can be buried horizontally in trenches, either in lengths of pipe or in coils (see Figure 5). The loop could also be placed at the bottom of a pond (see Figure 6) provided the pond does not freeze in the winter. This technique can be very efficient. A heat transfer fluid is circulated through the ground loop and the heat pump heat exchanger to complete its ground connection. The heat pump system uses a standard conditioned air distribution system of duct work that is used for other types of heating and cooling systems.

In addition to space heating and cooling that is provided during the winter and summer seasons, the excess heat from the ground source heat pump compressor can be used to heat domestic hot water when the heat pump is in use.

Basic economics
The incremental cost of a typical residential geothermal heat pump installation over a comparable air sink/source system would be in the range of US$2,000 to $5,000, depending on which ground connection is used. Pond type ground source heat pumps tend to be the least expensive of the systems. The installed cost of a complete residential ground source heating and cooling system would be approximately US$1,800 to $2,800 per ton as compared with a standard central heating and air-conditioning system at US$1,200 per ton. Although the initial cost is more, the annual savings can make geothermal heat pumps well worth the extra expense.

Wave and Tidal Power
Wave and tidal power is an emerging new energy technology. Wave power results from harnessing the energy that is transmitted to waves by winds moving across the ocean surface. Both Japan and England realised the potential of wave energy in the 1970’s, and began to develop methods for utilising this resource for power generation. Wave energy generation devices fall into two categories – fixed generating devices, and floating devices.

Fixed generating devices
Fixed generating devices are mounted to the ocean floor or shoreline, and have significant advantages over floating systems where maintenance costs are high. The most promising fixed generating device technology is the Oscillating Water Column (OWC), which uses a two-step procedure to generate electricity. As a wave enters a purpose-designed vertical column, it forces air upward under pressure and past a turbine. As the wave retreats, the air is drawn back past the turbine as a result of reduced air pressure on the ocean side of the turbine.

Internationally, there is much research into developing oscillating water columns, which require less stringent siting conditions. Latitudes between 40-60 degrees, which is where the highest concentration of wave energy occurs, are most suitable for siting of these devices. The west coasts of Europe and the U.S., and the coasts of New Zealand and Japan are particularly suitable for wave energy generation. China has already constructed a 3kW OWC shoreline device, which has an artificial gully and a Wells Turbine. India has built a 150kW OWC caisson breakwater device using Wells Turbine.

Figure 7 – The retreating wave sucks air back down the column past a turbine. The turbine turns in the same direction, irrespective of airflow.

In Australia, Energetech Australia Pty Ltd has produced a prototype model of a new type of oscillating water column. The system uses a parabolic wall which focuses wave energy into the column. Initial testing of a 1/25th-size model has been encouraging and Energetech has received a $750,000 grant through the Australian Greenhouse Office’s Renewable Energy Commercialisation Program to construct a 300kWp wave power generator on the breakwater at either Newcastle or Port Kembla in NSW.

Floating devices
Floating wave energy devices generate electricity through the harmonic motion of the floating part of the device. Unlike fixed systems that use a fixed turbine powered by the motion of the wave, the devices in these systems rise and fall according to the motion of the wave. Work on floating devices stalled during the 1980’s, owing to a miscalculation in the cost of energy production by a factor of ten, and it has only been in recent years that the technology was reassessed and and the error identified.

Conclusion
Part of the cost difference between renewable energy installations and traditional systems is actually illusory, a consequence of the way we have traditionally looked at energy costs. The use of conventional fuels carries along with it many costs, which are passed along to society at large, rather than to the direct consumer of the energy, thus making conventional energy sources appear more economical. Some of these costs that are currently being charged to society rather than to the energy consumer include: emissions from fossil fuel combustion, which contribute to acid rain, global warming and decreased air quality; the damage caused to environmentally sensitive areas due to oil spills from drilling or transportation of oil or gas; where nuclear energy is used, hazardous radioactive wastes from the generation of electricity must be transported and disposed of; even the cost of maintaining a military presence in the Persian Gulf in order to maintain secure passage for petroleum shipments.

By accepting these as costs that society in general must face, we are, in effect, subsidising the use of conventional fuels, thus making them appear more economical for energy users. However, as we face the problem of the continuing rise of fossil fuel energy costs, compounded by energy shortages in most third world countries (as well as affluent consumer societies), the cost of renewable energy systems no longer seems prohibitive.

This article has been prepared from information supplied by the North Carolina Solar Center, the Geothermal Education Office, U.S. Department of Energy, Australian Greenhouse Office, Australian Department for the Environment and Heritage, EnergyAustralia, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and ECNZ Geothermal Group.

Website Resources
American Bioenergy Association
www.biomass.org

American Wind Energy Society
www.awea.org

Australian Dept for the Environment and Heritage
www.environment.gov.au/minister/env/2001/index.html

CSIRO Atmospheric Research
www.dar.csiro.au/

Electricity Supply Association of Australia
www.esaa.com.au/

Energy Australia
www.energyaustralia.com.au/

International Geothermal Association
www.demon.co.uk/geosci/ecicelan.html

North Carolina Solar Center (NCSU)
www.ncsc.ncsu.edu/fact/11body.htm

Renewable Energy World
www.renewableenergyworld.com/index.html

Windpower Monthly
www.windpower-monthly.com

References
American Wind Energy Society
Economics of Small Wind Turbines.
Washington, DC, 1993.

D. Chynoweth and R. Isacson (eds.)
Anaerobic Digestion of Biomass
Elsevier Science Publishing Company, 1987.
(Available from Chapman & Hall Publications, Florence, KY, USA)

C.T. Donovan Associates, Inc.
A Sourcebook on Wood Waste Recovery and Recycling In the Southeast.
Southeast Regional Biomass Program, Department of Energy, Muscle Shoals, AL, June 1994.

C. Ross and J. Walsh
Handbook of Biogas Utilisation
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PH&G July/August 2001 / Issue 59


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