Introduction
We have seen there are advantages in producing energy by renewable means. But what sort of renewable source is best (or what combination)? We will give a description of each category of energy source beginning in this section with wind, geothermal, biomass and solar to get an idea of the energy potential of each source and future prospects.
Wind Power
Though air is rather insubstantial (the average density of air at NTP (20 C, 1 atm) is 1.204 kg/m3 ), air has a significant passive effect because there is so much of it. Whilst we are generally unaware of the fact, the entire weight of the atmosphere above us exerts the same pressure at ground level as would 10 m of water. Air is also capable of carrying energy by virtue of its motion, or kinetic energy. The wind speed in the UK will generally vary from 0-30 ms-1 (Table 1) and if one were to point a hoop of area 1 m2 into the wind, the energy passing through the hoop each second varies with the cube of the wind speed, reaching 16.2 kJ when the wind speed is 30 ms-1, equivalent to 16.2 kWh of energy each hour (Table 1).
| Beaufort Number | Wind Condition | Wind Speed (mph) | Wind Speed (ms-1) | Power in 1m2 (kw) |
| 0 | calm | < 1 | < 0.5 | < 0.00007 |
| 1 | light air | 1 - 3 | 0.5 - 1.4 | 0.00007 - 0.0016 |
| 2 | slight breeze | 4 - 7 | 1.5 - 3.2 | 0.002 - 0.019 |
| 3 | gentle breeze | 8 - 12 | 3.3 - 5.4 | 0.021 - 0.094 |
| 4 | moderate breeze | 13 - 18 | 5.5 - 8.0 | 0.1 - 0.3 |
| 5 | fresh breeze | 19 - 24 | 11.8 - 13.8 | 0.31 - 0.96 |
| 6 | strong breeze | 25 - 31 | 11.8 - 13.8 | 0.98 - 1.5 |
| 7 | moderate gale | 32 - 38 | 13.9 - 17.0 | 1.6 - 2.9 |
| 8 | fresh gale | 39 - 46 | 17.1 - 20.6 | 3.0 - 5.2 |
| 9 | strong gale | 47 - 54 | 20.7 - 24.1 | 5.3 - 8.4 |
| 10 | whole gale | 55 - 63 | 24.2 - 28.2 | 8.5 - 13.4 |
| 11 | storm | 64 - 75 | 28.3 - 33.5 | 13.6 - 22.5 |
| 12 | hurricane | > 75 | > 33.5 | > 22.5 |
This is a significant quantity of ‘free’ energy that could potentially be harvested. A wind turbine is a device for extracting energy from the wind and will typically consist of blades that rotate with the wind. The motion produces electricity by driving a conventional electrical generator.
A generator will typically extract 30-40% of the wind energy streaming through the area swept out by the blades. It is not possible to remove all the energy from the wind because it would then cease to flow.
A turbine actually works by moving a tube of air into a larger cone as shown in Fig. 1, and the process has a maximum efficiency of 59%, the Betz limit.
There are many different designs of turbine: horizontal axis, vertical axis, single blade, 2-blade, 3-blade and so on. The most common type is a 3-blade horizontal axis device (which will chase the wind by changing orientation).
The blades act as aerofoils (and are not simply pushed by the wind) to allow the blade tip to move faster than the wind speed and extract more energy. Fig. 2 shows a comparison between various devices (lift vs drag).
Figure 1: A turbine moves an area of incident air A1 at velocity V1 to a larger area. Conservation of mass requires that V1A1 =V2A2.
© UHI Owen Inger - Gray
Figure 2: Comparative performance of similar sized turbines. © A.MacKenzie.
Wind is unpredictable and there is not much energy when the wind speed is low. Large turbines are generally economically viable, but small devices tend to be very costly and perform poorer.
Choosing a site either onshore or offshore is a major difficulty because of impact and potential environmental damage which is a major barrier to the widespread deployment of wind turbines. It is also necessary to develop grid links to transfer energy from the often remote points of generation to market. Finally there is the issue of who ‘owns’ the wind energy and if there should be any payment for it.
Activity 1
(a) How do turbine blades capture wind power?
(b) If the mean wind speed over an area is 8.0 ms-1, how much power would a 3.6 MW Siemens wind turbine generate in a year? The power curve of the turbine is shown on the right.
Why is this calculation not absolutely precise? If the turbine costs £3.0 m to manufacture and install, what is the payback time assuming a wholesale price of electricity of £45 per MWh?
Solution
a.
Wind turbine blades work by generating lift due to their shape. The more curved side generates low air pressures while high pressure air pushes on the other side of the aerofoil. The net result is a lift force perpendicular to the direction of flow of the air.
The lift force increases as the blade is turned to present itself at a greater angle to the wind. This is called the angle of attack. Angle of attack is the angle between a chord line of aerofoil and the vector representing the relative motion between the body and the fluid through which it is moving.
b.
The graph on the below indicates the power output at a site with mean wind strength of 8.0 m/s would be 0.671 MW. The turbine should produce 671 Units per hour, or 0.671 x 24 x 365 = 5.9 million units per year. The wholesale value of electricity is assumed to be £45/MWh. There are plans to return the Contracts for Difference (CfD) late 2021 with onshore wind establishments over 5MW. This does not apply to this singular 3MW device. Renewables Obligation Certificates (ROCs) ended in 2017 and are no longer considered an accredited cost.
| POWER OUTPUT at 8.0m/s |
0.671 MW |
| UNITS PER YEAR | 0.671 x 24 x 365 = 5.878x106 Units |
| INCOME PER YEAR | 5.878x103 x (45) = £264,510 |
| NET CAPITAL COST | £3,000,000 |
| PAYBACK TIME | 3,000,000 / 558,410 = Approx. 11.4 Years |
The calculation is straight forward but should include interest rates on borrowing, rising electricity prices, grants, deviation in turbine power curve from ideal shape, maintenance, falling turbine costs etc. But the most significant problems are that average wind speed is not a good indicator of power generation, and the wind strength is significantly higher at the 80-100 m height of a big wind turbine than near ground level because surface friction slows wind down.
Geothermal Energy
Geothermal energy refers to energy stored in the ground. This can either be energy that was absorbed from the sun, or energy produced deep within the Earth arising from natural radioactive decay.
Fig. 3 shows the anatomy of the Earth. The core is very hot, about 4,000oC, and the mantle is 3,000oC in some places. Some of this heat conducts to the surface through the crust, and because heat will only travel along a temperature gradient, we find that the temperature of the crust gradually increases with depth (Fig.4).
Figure 3: A cross section of the Earth [source] (public domain)
Figure 4: The geothermal gradient beneath a continent, showing how temperature increases more rapidly with depth in the lithosphere than it does in the deep mantle. [Source] (fair dealing)
A geothermal heating and/or cooling system typically involves a production well to deliver warm water from the rocks beneath the site; a heat centre, pumps and pipework to deliver thermal energy to the end user; and a re-injection well to dispose of water and maintain resource pressure.
The production well will typically be drilled into the ground to a depth of 50–3000m where the rock contains large quantities of groundwater. Some project developers are considering wells as deep as 9000m, and as drilling technologies improve and costs come down this may become feasible.
When the hot groundwater reaches the surface, it passes through a heat exchanger which transfers most of the heat into a separate loop of pipe containing clean water. The temperature of this clean loop can then be increased further by use of a heat pump to match customer requirements. The heat pump can be centrally located in an energy centre, or located in each building, depending on the customers requirements.
After passing through the heat exchanger, the groundwater is piped to the injection well (or surface disposal facility) and pumped back into the ground (or treated before pumping into the sea). The injection well will typically reach similar depths to the production well, and be spaced some distance away from the production well to allow the groundwater to heat back up before returning back up the production well.
The spacing between the production and injection can be achieved by directional drilling from a single well pad to minimise surface pipework.
Though the temperature above ground varies through the day and throughout the year, the Earth’s crust is a huge storage system that maintains a temperature 5 metres below the surface equal to the mean annual temperature above ground. In Scotland, the average temperature is approximately 8.5oC. By sinking pipes into the ground, this can act as the nearly constant reference (or heatsink) of a heat pump. Moving heat with a heat pump is up to four times as efficient as generating the heat electrically or by combustion. If a heat pump were used to raise the temperature of a building by heating circulating water at 50oC , it would not be unusual to achieve Coefficient of Performance (COP rating or efficiency) of 3.5 to 4.5 in Scotland. This would mean a reduction in energy use and cost (after initial payback cost).
The difficulty is that the area under ground in contact with pipes must be large enough that the source is not depleted. Heat should be removed at the rate it flows back in from the surroundings. This generally requires that the reference loop should ideally be in contact with flowing water, or a stream could be used for the reference (open loop system).
Figure 5: Measured temperatures at 1 km below ground level in the UK [source] (public domain)
The Coefficient of Performance (COP) measures the efficiency of a heat pump and it does this by measuring the amount of power input compared to the amount of power output produced by the considered system. Hence, the higher the value, the more efficient the system is.
- Ground source heat pump: COP of 3.5 to 4.5
- Air source heat pump: COP of 2.5 to 3.5
- Water source heat pump: COP of up to 5
The system's actual efficiency can be calculated by the amount of work it has to do, given the difference between the outside and inside temperature. The closer the two environments are, the less work the heat pump has to accomplish in order to reach the desired temperature.
Activity 2
- List and explain the various types of geothermal energy sources?
- Why do ground source heat pumps require an antifreeze solution to transfer heat from the cold side of the heat pump to the warm side?
Solution
a.
Ground-Source Heat Pump
A ground-source heat pump extracts primarily solar thermal energy, and some geothermal energy, from the very shallow surface. Systems can be closed loop (pumping fluid through buried pipes) or open loop (pumping water through porous rocks via a production and injection well).
Surface-Water Heat Pump
Bodies of water on the surface, including lakes, rivers and even the ocean, can be great sources of renewable heating and cooling.
Minewater
Abandoned coal mines in Scotland’s central belt and across large areas of England and Wales are full of warm water. These legacies of the fossil fuel age can be recycled into vast sources of low-carbon heating and cooling at relatively low cost. Heat exchangers quantities of low carbon heating and cooling at low cost. Heat exchangers and heat pumps are usually used to elevate the temperature of the water to be delivered to the customer.
Aquifers
Aquifers are rocks with pore-spaces and natural fractures that exist across much of the UK generally, and in particular can overlap with onshore oil fields. Geothermal aquifers are often referred to as Hot Sedimentary Aquifers (HSAs).
Granites
Heat from radio thermal granites in the highlands of Scotland, the north east of England, and Cornwall can be converted into electricity by the use of Engineered Geothermal System (EGS) technology, with lots of heat as a useful by-product. This is often referred to as Hot Dry Rock geothermal, which targets naturally occurring fractures.
Disused Oil Well
When oil and gas wells are near the end of their economic life, they often produce large quantities of hot water. When local heat demand exists, or if a business which requires low cost heat (such as a heated greenhouse operator) wants to invest locally, the oil well can be given low-carbon geothermal afterlife.
CCG Well
Cyclic Circulating Geothermal (CCG) wells produce warm water and injects cold water via the same well, and can be deployed in any geology. This is an effective technological mitigation option when an aquifer or granite production/injection well encounters low flow.
b.
Remember a heat pump is just a refrigerator working in reverse. But if the working fluid is to draw heat from the cold reservoir, it must be colder that it because heat will only flow from cold to colder, and the greater the temperature difference, the more heat flow. The working fluid must therefore operate at temperatures well-below zero and has to be prevented from freezing.
Biomass
Biomass or biomatter is living or dead but non-fossilised organic material. Organic matter is the product of photosynthesis (Fig. 6) and organic compounds incorporate carbon dioxide that has been removed from the air.
Encouraging the growth of trees and vegetation is a form of carbon sequestration, a method of removing carbon dioxide from the atmosphere, that works so long as the plant matter is not allowed to decay as this process releases carbon dioxide back into the atmosphere.
Carbon offsetting is when trees or long-lived plants are deliberately grown to remove from the atmosphere the same quantity of carbon released by other processes. Forests are natural carbon sinks, but become less effective as the forests mature.
Figure 6: Photosynthesis [source] (public domain)
Biomass sources for energy include: (Fig. 7)
- Wood and wood processing wastes; firewood, wood pellets, and wood chips, lumber and furniture mill sawdust and waste, and black liquor from pulp and paper mills
- Agricultural crops and waste materials; corn, soybeans, sugar cane, switchgrass, woody plants, and algae, and crop and food processing residues
- Biogenic materials in municipal solid waste; paper, cotton, and wool products, and food, yard, and wood wastes
- Animal manure and human sewage
Though the act of growing plants and trees in an annual cycle for burning or to make synthetic fuel does not remove carbon from the atmosphere, the process is nevertheless carbon neutral because the quantity of carbon dioxide released by burning is the same as that removed from the air during the growing season. Biomass production for heating, transportation and electricity generation is therefore a sustainable method of energy use.
Figure 7 Types of biomass [source] (public domain)
Remembering that a hectare is an area of 100 m x 100 m, we would expect a crop grown for energy production to produce about 100 GJ (2.4 toe) of energy per annum. This energy can be recovered by incineration, but in some plants a significant fraction of the stored energy is conveniently in the form of vegetable oil that can instead be extracted and used as fuel. The amount of oil produced from a variety of crops is shown in Table 2, and it is clear some plants deliver significantly above the average. Other plants produce sugar instead of oil, but this can be converted to ethanol and also used as fuel.
The problem is that Scotland consumes the energy equivalent of 14 million tonnes of oil a year (approx. 160,000 GWh), and of the crops in Table 2 only rapeseed grows well in Scotland.
An area of 140,000 km2 would be need to be set aside to produce enough oil. This amount of land is just not available. There would be concern over how even a limited change of land used to grow crops for energy generation may actually be environmentally damaging.
Whilst biomass can contribute as one of a suite of measures, it can never be the entire solution to the carbon problem.
It is also possible to use farm waste (including animal matter) for energy production, but in the UK, fast growing trees with annual coppicing is the most common way of producing combustible biomatter.
Another common option is incinerating garbage, which has the added benefit that less rubbish needs to be buried in landfills.
| Crop Type |
toe/ hectare/ year |
Crop Type |
toe/ hectare/ year |
| maize | 0.15 | jojoba | 1.53 |
| sesame | 0.59 | jatropha | 1.59 |
| rapeseed | 1.00 | brazil nuts | 2.01 |
| olives | 1.02 | avocado | 2.22 |
| castor beans | 1.19 | coconut | 2.26 |
| pecan nuts | 1.51 | oil palm | 5.00 |
| copaifera langsdorffii | 11.01 |
Table 2: Vegetable oil yields of common energy crops (source) (CC0)
Activity 3
- What are the negative effects in relation to biomass energy?
- Draw a block diagram and explain the process for a direct combustion system of a typical biomass plant?
Solution
a.
Deforestation and Farming Practices
Biomass requires energy crops grown on a large scale. Grasses and other inedible, high-cellulose crops are the most common. many energy companies use forest timber for fuel and clear-cut mature trees that, if left untouched, remove carbon dioxide emissions from the atmosphere. Actions like these lead to deforestation, causing habitat loss, soil erosion, destruction of natural beauty, and more.
The removal of forest for the production of energy crops can also increase greenhouse gases; 25 to 30 percent of greenhouse gases released each year are a result of deforestation.
Mitigating these agricultural risks and impacts hinges on sustainable harvesting practices and responsible land use.
Water Use
Like coal and nuclear plants, biomass plants may disrupt local water sources. Water use at a biomass plant ranges between 20,000 and 50,000 gallons per megawatt-hour. This water is released back into the source at a higher temperature, disrupting the local ecosystem. The nutrient runoff from energy crops can also harm local water resources as well. Plants require water to grow; when energy companies grow trees and other crops for a bioenergy plant, they use a lot of water for irrigation. On a large scale, this exacerbates drought conditions, impacting aquatic habitats and the amount of water supply available for other purposes (food crops, drinking, hydropower, etc.)
Air Emissions
Despite being a relatively clean alternative to more harmful fossil fuels, burning biomass in a solid, liquid, or gaseous state can also emit other pollutants and particulate matter into the air, including carbon monoxide, sulphur dioxide, volatile organic compounds, and nitrogen oxides. In some instances, the biomass burned can emit more pollution than fossil fuels. Unlike carbon dioxide emissions, many of these pollutants cannot be sequestered by new plants. These compounds can lead to a number of environmental and human health issues if not properly contained. Filters, cleaner biomass sources, gasification systems and electrostatic precipitators can help the issue.
Transporting waste from forestry and industry to a biomass plant also carries a significant carbon footprint from the petroleum used by transportation. This release of greenhouse gases may be a secondary environmental impact from biomass energy generation, but it's important nonetheless.
b.
Direct combustion systems feed a biomass feedstock into a combustor or furnace, where the biomass is burned with excess air to heat water in a boiler to create steam. Steam from the boiler is then expanded through a steam turbine to produce mechanical or electrical energy.
Some extracted or spent steam from the power plant is also used for manufacturing processes or to heat buildings. These combined heat and power (CHP) systems greatly increase overall energy efficiency to approximately 80%, from the standard biomass electricity-only systems with efficiencies of approximately 20%
A simple biomass electric generation system is made up of several key components. For a steam cycle, this includes some combination of the following items:
- Fuel storage and handling equipment
- Combustor / furnace
- Boiler
- Pumps
- Fans
- Steam turbine
- Generator
- Condenser
- Cooling tower
- Exhaust / emissions controls
- System controls (automated).
Figure 9: Block Diagram of simple bimass electric generation system (© UHI Owen Inger - Jones, Lews Castle College)
Solar Energy
The sun emits energy over a wide range of frequencies. The mean integrated energy incident on a square metre of the Earth’s surface, the irradiance, is 342 W (though the figure is much lower in Scotland). Solar Panels are made of photovoltaic (PV) (Fig. 11) cells that are sandwiched between layers of silicone. The electronic properties vary in each layer and when hit by photons from sunlight, create an electric field. This ‘photoelectric effect’ creates the current that produces electricity, which is passed through an inverter that can be used by the National Grid or directly to the property the Solar Panels are linked to.
A solar panel tracking system (Fig. 10) is made up of an array mounted on a mechanical moving frame which allows the panels to follow the sun as it moves position across the sky, capturing an optimum amount of energy. Whilst solar trackers can increase the amount of electricity a system is able to produce, they are costly to make and because of this they tend to be found in larger scale solar installations rather than domestic. There are two main types of solar tracker, single axis and dual axis.
With advancements in solar energy technology, the cost of solar panels has fallen drastically over the last few years (approx. £1,000 per m2 in 2008). Nowadays a single solar panel costs £150-£400 per m2, but can vary depending on quality and type of system. The table below provides a breakdown of PV cells taken from a range of companies. It is a broad estimate so figure may vary according to size, type, and quality of the system.
|
System Size |
Estimated Cost (£) |
Number of Panels |
Roof Space (m2) |
|
3kW |
4,000-6,000 |
12 |
22 |
|
4kW |
5,000-7,000 |
16 |
29 |
|
5kW |
6,000-8,000 |
20 |
32 |
|
6kW |
7,000-9,000 |
24 |
43 |
But the energy in sunlight does not need to be converted to electricity; a solar hot water system is a relatively cheap way of trapping solar energy and storing this energy as heat. It is then used as required for showers, baths, cooking and so on. Installation is a standard plumbing exercise with only an additional heating coil needed in the tank or cylinder (though there may also be an automated control system for the pumps and valves). Fig. 12 shows a typical solar hot water system. The panel is placed on the roof facing south and can be fitted like a Velux window. There are two main types of solar water heating systems - active, which uses a pump to circulate the water between the tank and the collectors, and passive, which relies on natural convection to circulate the water.
Figure 12: Active (Upper) and Passive (Lower) solar water heating system [source] (CC0)
Active systems can be either direct circulation or indirect circulation. Direct circulation systems circulate domestic water through the collectors and to the storage tank. These are best-suited for mild climates where temperatures rarely drop below freezing. Indirect circulation systems circulate a non-freezing heat transfer fluid through the collectors and then through a heat exchanger in the storage tank. These are preferred in cold climates where the pipes in a direct circulation system might freeze.
Passive systems are usually less expensive but less efficient. They can be either integral collector/storage systems or thermosyphon systems. The integral collector/storage type is typically used to preheat water for a conventional water heater, and is best-suited to climates where temperatures rarely fall below freezing. Thermosyphon systems rely on natural convection to circulate the water, so the tank must be located higher than the collector panels - the heated water from the panels flows upward to the tank and the cooler water returns to the collector for heating.
The cost-effectiveness of a solar water heating system depends on a number of factors;
- Hot water use; the more hot water you use, the more likely a solar water heating system will pay for itself over time. They are usually most cost-effective for larger families or homes with a high demand for hot water.
- Cost of the system; passive systems are typically less expensive, but may not be practical or appropriate in many cases.
- Amount of available sunlight; solar applications obviously work best in locations with plenty of available sunlight. Ideally, collectors should be exposed to direct sunlight for the maximum possible number of hours each day, so proper location is critical to achieve optimum performance.
Activity 4
(a) Solar hot water systems cannot track the sun. Is this a problem with retrofits to existing homes?
(b) How is the output of a PV system described? Explain the I-V Curve?
Solution
a.
The diagram shows the daily movement of the sun across the sky as it varies with season. The reason for this curious path is that the Earth is tilted by 23.5o off the orbital plain of rotation about the sun, to which should be added to the latitude of the observer. A solar energy collection system should therefore face south and the particular angle is determined by the time of year when the energy is most needed. For example, if most of the energy is needed in high summer, the energy collector is only tilted by a small amount. For solar water heating systems, it is most likely that energy is needed in autumn, winter and spring, but there is very little heat in the sun in winter. If there is no way of moving the collector, the preferred orientation is therefore perpendicular to the path of the sun in autumn or spring.
Figure 13: The seasonal variation in the daily path of the sun across the sky (© UHI Owen Inger - Jones, Lews Castle College)
Fixed and tracking Photovoltaic cells.
[source] Used under fair dealing.
In a new build, the building should be aligned towards the south and the pitch of the roof should match the optimal angle. It is unlikely that an existing building will be set in this way making a retrofit very difficult.
b.
Figure 14: I-V Curve of a Solar Cell (Owen Inger - Jones, Lews Castle College)
The power of a PV cell is measured in kilowatts (kW) or kilowatts peak (kWp), the rate at which energy is generated at peak performance in full sunlight. Solar Panels, however, react to the visible light in the spectrum, so if it’s light enough to see, they will be generating electricity.
The power curve (right as two examples) has a maximum denoted as PMP where the solar cell should be operated to give the maximum power output. It is also denoted as PMAX or maximum power point (MPP) and occurs at a voltage of VMP and a current of IMP.
A current-voltage (I-V) curve shows the possible combinations of current and voltage output of a photovoltaic (PV) device. A PV device produces its maximum current when there is no resistance in the circuit, i.e., when there is a short circuit between its positive and negative terminals. This maximum current is known as the short circuit current and is abbreviated ISC. When the module is shorted, the voltage in the circuit is zero.
Conversely, the maximum voltage occurs when there is a break in the circuit. This is called the open circuit voltage (VOC). Under this condition the resistance is infinitely high and there is no current, since the circuit is incomplete.
These two extremes in load resistance, and the whole range of conditions in between them, are depicted on the I-V curve. Current (Amps) is on the (vertical) y-axis. Voltage (Volts) is on the (horizontal) x-axis.
The power available from a photovoltaic device at any point along the curve is just the product of current and voltage at that point and is expressed in watts. At the short circuit current point, the power output is zero, since the voltage is zero. At the open circuit voltage point, the power output is also zero, but this time it is because the current is zero.
There is a point on the knee of the curve where the maximum power output is located. Where if a line is drawn down from PMAX of the solar cell until it meets with the IV Curve. This is the point for consideration. Therefore;
The maximum power; MPP = VMPP x IMPP.
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