Solar Energy Flashcards
KWH calculation
Power is the rate of transfer of energy: Power = energy / time P = E / t
When the equation is rearranged, the expression for energy is:
Energy = power x time
E = P x t
Where: E = Energy in kWh.
P = Power in kW.
t = Time in h.
Solar energy available in the UK per year
Total energy = solar energy received per m2 per year x total area in m2 This equation can be expressed in symbol form: ET = S x A Where: ET = Total energy in kWh. S = Solar energy received per m2 in one year in kWh/m2/year. A = Total area of land in m2.
Solar thermal heatin
There are three ways in which this heat transfer fluid can then be used, all of which are on the AS Unit 1 Environmental Technology specification: 1. Active solar heating. This technique involves the use of a solar collector to absorb the Sun’s energy which is then typically used to heat domestic hot water (DHW); uses of DHW include heating homes, hot water for washing dishes, showers and bathing.
- Passive solar heating. This technique involves the direct absorption of the Sun’s energy into a building which can then circulate this energy naturally (or aided by electrical fans and ventilation systems) around the building environment. Air is typically the fluid heated and circulated around the building space as is the case in homes and workplaces. LO16 of this chapter illustrates some of the many passive techniques than can be implemented to reduce fossil fuel and electricity usage.
- Solar thermal engines. This technique involves use of a solar collector to heat a fluid much like the active solar heating technique described above. They differ however in that they are implemented in higher temperature environments where steam is produced, either directly from the heated transfer fluid or via use of a heat exchanger, which can then be used to drive a generator to produce electricity.
Components in a solar thermal system
- A solar collector.
- An energy transfer system (via a heat transfer fluid).
- Storage tank.
The solar collector, which may be a flat plate collector or evacuated tube (see LO4), captures the energy from the Sun and transfers this to a heat transfer fluid; this fluid may be water or an antifreeze depending on the climate at the proposed location. This fluid is circulated through the system, transferring the thermal energy to the storage tank. In mild and colder climates this storage tank will be supplemented with an immersion heater which can boost the DHW temperature when required as is the case in the UK and Ireland. It should be noted that actual systems contain many additional components and features to prevent freezing, prevent excessive pressure of the heat transfer fluid, prevent circulation in the wrong direction among other things
Direct or indirect loop systems
The heat transfer fluid outlined above can be water which is circulated through the solar collector, heated and then circulated through to the storage tank. This is the case in a direct loop system where the water in the storage tank has been heated directly via solar radiation.
When an alternative heat transfer fluid is utilised, which may be an antifreeze, the heat transfer fluid passes through the solar collector where it is heated and is then circulated to the storage tank. However, in this storage tank the energy from the heat transfer fluid is passed to the water in the tank via a heat exchanger. Hence in this system the water in the tank is heated indirectly.
Passive or active systems
The means by which the heat transfer fluid is forced to circulate differs in solar thermal systems. The oldest, and indeed most basic system is called a passive system.
The heat transfer fluid is forced to circulate due to natural convection currents set within the fluid when heated by the Sun. When the fluid is heated it expands and its density decreases. A less dense fluid rises, and a natural convection current is set up within the system as it is replaced by cooler more dense fluid which sinks. Solar thermal panels, that make use of this passive convection current method, are commonly called thermosiphon systems. This passive system has a number of notable benefits:
- Where high solar radiation occurs the heat transfer fluid receives energy more quickly and its temperature rises rapidly. Therefore, the flow rate due to convection currents is greater. This ensures that the energy from the collector is rapidly transferred to the storage tank as required on a hot day. Where low solar radiation occurs the rate of flow of energy through the system due to convection currents is lower. Consequently, in higher solar radiation levels the flow rate of energy is higher and lower when the solar radiation levels decrease. This correlates perfectly with the required flow rate of the system.
- No pumps are required reducing initial costs and running costs. This has a secondary benefit in that such systems can be deployed in regions where electricity supply may be limited or indeed non-existent.
- The system tends to have less maintenance requirements.
Usage of solar thermal in NI
The benefits of solar thermal systems in warmer climates are obvious with excellent solar radiation values which are more consistent throughout the year. They also lend themselves to the application of the more simplistic and cheaper passive systems that rely on the natural convection of the heat transfer fluid through the system. Their popularity is particularly high in nations including Israel, Spain and Greece.
However, the colder climate of NI presents challenges. The systems are active and require the use of a pump. They are also indirect requiring an antifreeze heat transfer fluid to prevent freezing during colder days and nights in winter. Their popularity has increased greatly in recent years as evident from the number of roofs with flat plate collectors and evacuated tubes.
Current usage of Solar PV in NI
Solar PV has been utilised extensively in warmer climates with high solar radiation values to produce electricity. They also can be extremely effective when used for the following:
- Remote homes where connection to the grid may not be possible or expensive.
- Communications satellites in space.
- Communications and remote monitoring of infrastructure on Earth (Such as Waste Water Treatment Works).
Direct radiation
This method, as the name implies, involves sunlight that travels from the Sun in a straight line directly to the surface of the Earth. Because the rays travel in a straight line, and parallel to each other, they can be blocked by an object and thus can create shadows. Direct radiation can be seen clearly in Figure 8.9 below where the Sun’s rays are moving through gaps in the trees; however, observe the regions directly under the tree to the right of the Sun. Here, direct radiation is not striking the ground as a shadow is formed. Yet light must still be striking this region, otherwise how can one tell that the grass is green or exists there. The light coming from this region must have reached it initially by a different method.
Diffuse radation
All the light striking the Earth’s surface cannot be accounted for by direct radiation. As sunlight strikes the Earth’s atmosphere some of it is scattered in different directions as it interacts with the molecules in the atmosphere. Some of this scattered radiation, travelling in all directions, does strike the surface of the Earth. These randomly orientated rays of radiation make up the diffuse light which can be substantial depending on weather, cloud cover and pollution levels.
It should be noted that even on a clear sunny day with the Sun directly overhead, diffuse radiation can still make up around 15% of the solar radiation striking a point on the Earth’s surface.
Reflected radiation
As the name implies, this radiation strikes a surface on the Earth and is reflected. This reflected light can then go on to strike another part of the Earth, building or even a solar panel. This reflected radiation is ultimately derived from either direct or diffuse radiation.
Disadvantages of Single Axis Tracking
- Complex electrical and mechanical equipment is required for the tracking system. This drives up the price of such systems.
- The motors or pneumatic cylinders required to rotate the collectors require power to operate. Consequently, some of the electricity produced by the system is used for the tracking operation.
- The locations where single axis tracking can be installed are restricted to high solar radiation regions. Tracking only maximises direct solar radiation which is consistently high in hotter climates close to the equator. This makes them unfeasible in NI.
- When one panel rotates towards the Sun in the morning and evening, a shadow will be created in behind the panel. A panel beside it will need to be spaced further away to ensure that this shadow region does not block the Sun from striking its collectors.
Advantages of Single Axis Tracking
- The energy output from the system will be higher; it can be higher by as much as 25%. In warmer climates this justifies the higher initial costs.
- The higher output obtained means that the repayment period is reduced which makes them better long-term investments in warmer climates.
Disadvantages of Dual Axis Tracking
- Complex electrical and mechanical equipment is required for the dual tracking system. This increases the price of such systems so that they are more expensive than the single axis tracking system.
- The motors or pneumatic cylinders required to rotate the collectors require power to operate. Consequently, some electricity produced by the system is required to power the tracking operation. The quantity of energy required for its operation is therefore larger than in the single axis tracking system.
- The locations where dual axis tracking can be installed are restricted to high solar radiation regions. Tracking only maximises direct solar radiation, which is consistently high in regions near the equator. This makes them unfeasible in NI.
- Over shading can occur in the east west plane and the north south plane for a dual axis tracking system. Consequently, less panels can be installed per unit area compared to single axis tracking systems.
Advantages of Dual Axis Tracking
- The energy output from the system will be much higher than a fixed system by as much as 35%. In warmer climates this justifies the higher initial costs.
- With outputs much higher the repayment periods are reduced which makes them better long-term investments in warmer climates.
Flat Plate Collector
Cold water is pumped into the collector at low level. Here it enters and travels up heat absorbing riser tubes. Water enters at low level and rises up the collector to correlate with natural convection currents which also rise upwards. The heat absorbing riser tubes are connected to an absorber plate, sometimes on top of it, sometimes under it; it has a special black paint applied which maximises the system’s absorption of solar radiation. Under this absorber plate is insulation which reduces the heat losses from the system through conduction and convection.
There is a glazing sheet cover on top of the panel. This glazing sheet is designed to:
1. Increase the solar radiation transmitted through it to the absorber plate.
2. Reduce as far as possible the heat losses back through the glazing sheet cover through conduction and convection.
3. Protect the system from rain and weather conditions.
Advantages of a Flat Plate Collector
- It is the cheapest collector available which is its main benefit.
- Is it suited to colder environments where snowfall is likely. This is because the snow can be melted due to conduction and convection heat losses from the panel.
- The efficiency of converting solar energy into thermal energy is good at approximately 80%.
Disadvantages of a Flat Plate Collector
- Even with the use of insulation to minimise the heat loses, they do exist, through conduction and convection within the flat plate collector.
- In higher temperatures the collector efficiency drops considerably especially over 30°C.
- Installation requires at least two people.
- As the collector surface is flat it will only be facing the Sun directly (angle of incidence = 0°) once per day.
- The weight of the panel filled with the heat transfer fluid can impose considerable loads on the roof surface and structure.
Evacuated Tube
The operation of an evacuated tube solar collector is more complex than that of a flat plate collector. This collector type is made up of a large number of individual tubes connected to the copper manifold heat exchanger, for example twenty tubes. The operation of a single evacuated tube is as follows:
- A double walled glass circular cylinder, called the evacuated tube, contains an absorber plate connected to a copper heat pipe as shown in Figure 8.20. This double walled evacuated tube has a vacuum between the two glass walls, which reduces heat losses via conduction and convection to almost zero.
- The absorber plate is attached to the copper heat pipe. As solar radiation strikes the absorber plate, energy is transferred to the copper heat pipe.
- The copper heat pipe has an enclosed cavity which is partly filled with a heat transfer fluid in liquid form. This fluid in NI is a mixture of water and glycerol to reduce its freezing temperature.
- When the absorber plate transfers energy to the copper heat pipe this heat transfer fluid is heated. At the lower end of the copper heat pipe, the heat transfer fluid increases in temperature evaporating into a gas which rises up the tube because of convection.
- The gas reaches the top of the copper heat pipe, which is embedded in the copper manifold heat exchanger, which has water circulating through it.
- The high temperature vapour loses energy to the colder water in the copper manifold heat exchanger. The vapour condenses back into a liquid when sufficient energy is transferred to the water.
- The heat transfer fluid, now in liquid form, returns to the bottom if the copper heat pipe under the action of gravity
- This cycle, incorporating steps 1-7, continues provided solar radiation is incident upon the evacuated tube
Advantages of an Evacuated Tube
- Its circular geometry results in the tube facing perpendicular for most of the day
- Heat losses through conduction and convection are almost zero
- There is less loading onto roof and support structure as the system is not filled entirely with a heat transfer fluid
- It has higher efficiencies, of as much as 90%, that can be maintained at very high temperatures
- They can typically be installed by one person
- As they can be stored vertically, transportation costs can be minimised
Disadvantages of an Evacuated Tube
- It has higher cost compared to a flat plate collector
- Where snowfall is possible, the evacuated tube is not able to melt snow as easily due to lower heat losses through conduction and convection
Parameters effecting the area of solar collectors required
Solar radiation levels; The solar radiation values differ substantially at various locations on the Earth’s surface. A location near the equator has very high solar radiation values and thus would require a smaller sized solar collector area to meet a family’s needs. Conversely, for a location much further north (or south) such as the British Isles, for the same family, a larger area of solar collectors will be required due to the much lower solar radiation values
Shading; One must not just consider the solar radiation values from online maps but also the unique location itself. If the proposed solar collector site has sunlight blocked at certain times of the day or year, by obstructions such as trees or buildings, then a larger solar collector area is clearly required
Family size; It is logical that a larger family will require more hot water on a daily basis. Therefore, a larger family will require a larger family will require a larger solar collector area compared to a smaller family
Benefits of a solar thermal collector
Reduced costs; Domestic hot water can be derived from the Sun, while households without such a system will have to pay for some form of fossil fuels to heat the water. Depending on the location of the house, the fuel type used may be from heating oil if the house is not connected to a main gas line, natural gas where it is available, and electricity from the immersion heater
Energy security; When a household installs a solar thermal system, they are protecting themselves from potential fluctuations in energy prices. The price of electricity will over time rise steadily by at least inflation, as the costs of goods and services increase on an annual basis. But crude oil and gas prices are much more volatile and can change considerably in a short time frame, due to factors such as turbulence in geopolitics. A family with a solar thermal system can protect themselves from these excessive price fluctuations and so increased their energy security
Benefits of a flat plate collector
“State the main benefit to a household of installing flat plate collector.” The use of the term ‘main’ could be to compare the flat plate collector to the evacuated tube. The primary advantage of the flat plate collector compared to the evacuated tube is cost. While it is less efficient, it costs less
