Capacity of a physical system
The project being undertaken is “An investigation of solar heating systems design”. It is a research project that requires evaluation and analysis of previous studies that have been conducted.
Energy is defined as “The capacity of a physical system to perform work while obeying the laws of conservation(Jones, 2009). Energy plays a very important role in everyday life as you need it to do everything from providing electricity, heat, sound and vision. There is not one specific type of energy it can come in various forms such as thermal, radiant, mechanical and electrical. Energy does not also come from one particular source but may come from a variety of sources which tend to be split into two groups known as Renewable and Non-Renewable energy sources. Fig.1 shows the types of energy sources available in both renewable and non-renewable forms. Renewable energy sources are those that are of infinite amount. Non-Renewable Energy sources are those that are of definite amount and eventually will run out.
Oil, Gas and Coal are known as the fossil fuels. Fossil fuels are found in the crust on the earth's surface. They are the products of millions of years of very high pressure and high temperature breaking down the fossils of dead plants and animals into their simplest forms of hydro-carbons.
The twentieth century saw the rise of the use of coal and oil. They became extremely important energy sources. Following that natural gas became a household name and started to be used extensively. Oil, gas and coal are believed to account for more than eighty-five percent of the worlds energy demand (Fost, 2009). This is an extremely high percentage and shows that they are being used at an extremely higher rate than there is available and is shortening the life-time of the fossil fuels. Fossil Fuels have the advantage of being cheap, easy to transport, comparatively low start-up costs, and have a straightforward combustion process (Shepherd & Randell, 2009). However, there have been major downsides to the use of fossil fuels.
Climate change has been a big topic of debate at the beginning of the twenty-first century as it has a direct effect on our planet. This has been a key issue for the leaders of the G20 to start to think about the impact it's making now and the changes that need to be made to prevent a disastrous future (Goldenberg, 2009). Fossil fuels are seen as big contributors towards global warming. The burning of fossil fuels produces gases such as carbon dioxide which is a leading greenhouse gas contributes sixty-one percent of the greenhouse effect (Quasching, 2005). The greenhouse gases affect the atmosphere and o-zone layer resulting in global warming and changing weather patterns. Global warming can cause much devastation to the earth as it has negative side effects on agriculture. Developing countries which rely extensively on farming to earn money will be affected by the change in weather patterns.
Political and Economical Issues can also affect the prices of fossil fuels. In the event of economic downturn one will notice the increase in price for fossil fuels.
Lastly, a major disadvantage is that fossil fuels will run out. They are being consumed far greater than being produced and it is becoming harder to find fossil fuels closer to the earth's crust and it is becoming more dangerous and expensive to harness.
For these reasons, it has become important to plan for future generations and start utilising renewable energy sources such as solar power to form heat and electricity. It will not be easy in the long run, but if we are able to gradually reduce the use of fossil fuels while being able to meet the world's supply and demand for energy it can prove worthwhile.
Solar energy is the energy provided by the sun. The Sun is the most powerful energy provider in the solar system. The sun is made up of approximately ninety-four percent (NASA, 2009) hydrogen atoms secondary comes helium atoms. Inside the sun we have the core which is of plasma state. In here we have constant atomic reactions known as a fusion of hydrogen atoms. This is due the atoms being forced together under very high temperatures thus eventually forming helium atoms. During fusion small amounts of mass is lost which is then converted into huge amounts of energy. The surface temperature of the sun is around 6000⁰C (Solar Trade, 2009) and contributes towards the solar radiation that we receive on earth. The radiation that we receive on earth is only a small fraction of the large radiation produced; however this small fraction can be in the region of 1.8x10¹⁴kWh (Goswami, 2007). This is around ten times more than the yearly global demand. The Sun provides earth with enormous amounts of solar radiation which is used in various ways. Fig.2 shows the various ways in which solar radiation is harnessed and used.
Solar Thermal Technology
Solar thermal can be defined as “energy generated from heat and employs heat directly to heat water, to raise the ambient temperature in buildings or to create steam which powers electricity generators (Busines Source Complete, 2009)
Solar thermal technologies have different applications. Several are listed below:
- Swimming Pool heating
- Domestic hot water heating
- Solar cookers
- Heating buildings
The designs of harnessing the solar radiation are different in any case and it can either be active of passive. Active systems are those that use solar collectors to harness solar radiation, and passive systems are inbuilt into the architecture itself while either using direct or indirect methods to heat the building. The main element of active solar heating is the solar collector. The absorber of the solar collector absorbs the solar radiation and the energy is transferred to the liquid. What one always aims to achieve from designing a solar collector is to enable it to retain as much solar radiation as possible and to achieve low levels of reradiation.
For one to design a successful solar system several feasibility points need to be taken into account such as technical, practical, reliability, cost, and environmental aspects. The design need not be too complex and must remain as simple as possible.
Aims and Objectives
The aim of this project is to critically assess the efficiency of solar heating systems design.
- To evaluate and analyse previous work done by various authors using different sources of media available.
- To consolidate the data to form concise summaries, comparatives and conclusions.
- To show by means of qualitative, quantitative and illustrative data the principles behind solar heating systems design.
- To propose modifications to current designs for an increased efficient systems.
To conduct this project it is important to understand the principles behind harnessing solar energy and it's conversion into thermal energy for useful applications, evaluation of thermal performance, evaluation and improvements to efficiency of the systems.
Active solar systems have solar collectors incorporated into the design to utilise the solar radiation. The two most commonly used solar collectors are flat-plate collectors or evacuated tube collectors. The basic outlay of a Flat-Plate collector consists of a transparent cover, black absorber plate, insulation and the collector box. The arrangement of an evacuated tube collector consists of a glass tube, an absorber plate with selective coating and collector tube. Between the collector tube and outer glass tube is a vacuum. Comparing the two types of collectors evacuated tube collectors can obtain higher energy even in cooler months and also require smaller collector area. However flat-plate collectors are much more economically viable and have simplistic designs (Rabl, 1985).
Based on research by T.Agami Reddy, he discusses several improvements that can be made to solar collector performance. The performance of solar collectors is determined by three things. They are the constructional components (design and materials), climate and operational features (fluid temperature, flow rate). Traditional materials used for the absorber plates on flat-plate collectors are copper aluminium and steel. Improving the glass cover by treating it can increase efficiency of the flat plate collector. One can increase efficiency by up to four percent (Goswami, 2007)by using low-iron glass material.
Further improvements can be made to the absorber plate by having a secondary glass cover to reduce infrared losses to external areas. This method could prove to be costly and result in a low optical efficiency. There are three ways in which solar collectors can gain energy convection, conduction and radiation. Convective, conductive and radiation losses are common in hot water heating applications and one can decrease the losses by coating using selective coating.
Reducing convective losses that occur between the absorber and transparent cover can also prove beneficial. Rather than using traditional materials honeycomb matrix can be used instead. The honeycomb materials can be either reflective or transparent. This can improve efficiency by up to twenty-five percent and double gain in passive trombe wall designs (Peuportier & Michel). Although there is a great benefit a great disadvantage of this material use is the poor aging of the product.
Peuportier and Michel conducted a study in France to analyse the achievement of high solar fraction in solar housing for which investment cost were limited. Six houses were subject to experiment, two with active systems and four with passive systems. Two types of transparent covers were compared which were a capillary structure and a simple polycarbonate plate. Their results concluded that the transparent cover was overall more efficient however the thermal comfort was not reduced by either passive or active systems.
Achieving an improvement in the solar collector performance may seem difficult but it can be done in a very simplistic design. To increase the efficiency side-reflectors may be added to the flat-plate collector. This is a cost-effective solution that gives and increased efficiency while maintain a simple design. Simplistic design is one of the key features required in design a solar heating system. These ideas behind solar system improvement that T.Agami Reddy proposes can lead to even more effective and efficient flat-plate collectors.
Evacuated tube collectors are seen as being very efficient and are able to achieve high temperatures. In comparison to flat-plate collectors they can even achieve high temperatures in the winter season. They are also beneficial because they do not disrupt the aesthetical features of a building and can be incorporated into a building's design. However with that aside improvements can be made to their durability, reliability and manufacturing costs. The can be expensive so there still is an issue with the cost effectiveness of the system. All they may achieve a high output and do what they are set out to do the start up costs can be high (Ramlow & Nusz, 2006).
The design of passive solar systems is taken place in the architectural design as the system is incorporated into the building rather than put on top of the roof like a active system design. Passive systems are applied to housing as well as commercial sector for the use in public buildings. Distributing heat in residential housing can be easier than that of a commercial building. Some commercial building may have a lot of heat coming from other sources such as machines and natural body heat. “According to a survey of the energy consumption of public buildings in the state of Baden-Wuerttemberg in Germany the average consumption of heat is 217 kWh/m²a, with an average electricity consumption of 54 kWh/m²a. The specific energy consumption of naturally ventilated office buildings in Great Britain is in a similar range of 200-220 kWh/m²a for heating and 48-85 kWh/m²a for electricity consumption (Eicker, 2006) .” The amount of heat consumed by commercial buildings does depend on what the building is used for. To regulate the heat supplied to a building a temperature control system can be incorporated into the passive design.
Passive solar heating design systems can vary according to what region of the world you're in, the size of your house and the climate of the area. What one may always try to achieve is to maximize solar gain in the winter and have minimal solar gain in summer. A problem with passive solar systems design is that the thickness of external walls can cause losses due to space. A way to overcome this problem could be to have a vacuum insulation system incorporated into the building. However, transmission losses still account for sixty to eighty percent (Santamouris, 2003)of all thermal loss. For passive solar heating systems to be efficient one has to take into account the building itself and it has to be able to function with the passive system. Early passive heating systems where inefficient and overheated houses because they couldn't cope with the designs of the house. As a lot of houses we currently live in were built many years ago it would be difficult to ensure that a passive system would work efficiently if incorporated into the design. One would have to ensure the house is well insulated and south facing first before incorporating the passive system into the design. However it can easily be incorporated into the structure of new builds as the right materials and the right positioning of the house can be used to achieve a high efficiency of the passive heating system (O'sullivan, 1988).
The improvements being made to solar heating systems design are extremely important however it is not beneficial unless they are actually used more. Currently leading countries in the solar thermal market are China, India and Japan. Between them they hold seventy-five percent of the market shares. According to research conducted by ABS Energy research, “The European total increased to 10million m² in 2001. The strongest growth in Europe is expected to come from Germany and Italy followed by France, the Netherlands, Portugal, Spain and the UK.”
Currently, harnessing solar energy for heating systems is moving forward however there are still problems that need to be addressed. The application for which the solar heating system is used still depends on a variety of things such as economical, social and environmental conditions as the use of the systems heavily depends on the climate. With the help of improvement and advancement it the technologies used to build such systems, scientist and engineers are able to improve on past designs to allow for a more efficient and economically viable product. Harnessing solar energy can make big contributions with regards to energy supply and demand especially in developing countries and on continents who receive high solar insolation throughout the year. Solar heating systems are not only concerned with heating water but with heating a whole building. All buildings have solar radiation hitting them from an angle and to benefit from this radiation the buildings should be constructed to profit and utilities it the best of their ability (Chwieduk, 2004).
Active Solar Heating Systems
Active solar heating is the use of fans, pumps or some type of mechanical or electrical method to capture solar radiation and convert it into usable heat. The most common use of this type of solar heating system is for the use of solar water heating. The focus on active heating systems for this report will be based on solar hot water systems.
The underlying ideology behind solar hot water heaters can be applied to other systems that use active heating technology.
Flat Plate Collector
A flat-plate collector is a heat exchanger that converts heat gained from solar radiation into useful energy for applications such as hot water and space heating, industrial processes and pool heating. Flat plate collectors are the most commonly used collectors around. A flat-plate collector is made up of different components. Flat-plate collectors can operate from temperature values ranging from 150-200oC (Quaschning, 2010). Flat-plate collectors vary in size but they typically come in sizes around 4 inches wide, 8 inches long and 4 to 6 inches deep (Benjamin, 2006). Figure 1 shows the major components within the flat-plate collector. They consist of the transparent cover, absorber plate, tube array, insulation and the collector box.
The process in which the flat-plate collector works is that the absorber is heated via the transparent cover by solar radiation. The absorber plate then heats the fluid in the tube array which supplies the source with heat energy via some type of fluid. The transparent cover and the insulation beneath the absorber plate help to prevent thermal losses from the flat-plate collector to the environment. The absorber component of the flat-plate collector can be very complex. There are varied configurations in which the absorber can be positioned. The absorber is designed to suit both air and water fluid type absorbers. Absorber plates are usually made from copper, aluminium and steel to which it is either coated in a black paint or a selective surface is applied. The amount of solar energy the absorber plate can absorb does depend on the type of coating. Electrolytic and chemical treatments are used to produce absorber plate surfaces with high values of absorbance and low levels of emission.
A typical selection for a selective surface is made up of a highly absorbing thin upper layer. Efficient absorber plates should be able to convert solar radiation to a heat source with minimal energy losses. The solar absorber can be referred to as tandem absorbers. That is they are made up of layers with different optical properties. The top layer is commonly made up of a semiconducting coating that is able to absorb solar radiation in high quantities. The bottom layer is made up of reflecting metal material such as the ones stated earlier (Kalogirou, 2004).
The glazing material aids in the utilisation of solar energy for a flat-plate collector. The most vital qualities required for glazing materials are reflection (ρ), absorption (α), and transmission (τ). The absorptance of a material is the ability of the substance to absorb a certain fraction of solar radiation. Reflection refers to the ability of the material to reflect solar radiation and transmittance refers to the ability of the material to transmit solar radiation through transparent objects. The main principle behind the glazing materials is that reflection and absorption should be low values and transmission should be a very high value to allow for maximum efficiency achievement.
There is a relationship that connects the reflection, absorption and transmission energy this is written in accordance to the laws of conservation of energy (P. Rhushi Prashad, Byregowda, & Gangavati, 2010):
Absorption, refletion and transmission all vary with temperature, surface qualities and material. The major requirements of the glazing material is to allow as musch solar insolation as possible and keep the thermal losses to the surroundings to a minimum.
The glazing material typically used for flat-plate collectors is glass. Glass is chosen as a suitable material as it can transmit up to 90% of shortwave solar irradiation. Low iron glass in particular can achieve transmittance values in the region of 0.30 to 3μ (P. Rhushi Prashad, Byregowda, & Gangavati, 2010).
Plastics and films sheets are another material considered for the use of glazing materials. They have the ability to transmit high short wave radiation however most of their transmittance values lie in the middle of the thermal radiation spectrum and thus may allow long wave transmittances. Plastics and films are not used for the reasons that they are not suitable for the temperature ranges that the flat-plate collector might reach and will therefore be more susceptible to degrading than a glass counterpart (Kalogirou, 2004).
Transmission can be improved if antireflective coatings are used. The glazing used must be able to allow as much solar irradiation in as possible and keep the loss of heat to a minimum. Glass is a good material to use for the glazing material however, absorption of long wave radiation can cause the temperature in the glass to increase and thus heat is lost to the surroundings by either radiation or convection (Kalogirou, 2004).
Glazing materials provide an insulating effect to the fat-plate collector. It does this by keeping heat loss by convection to a minimum. Dirt or dust on the glazing material does not affect the operational function of the flat-plate collector by large amounts, transmittance values may still stay up to 4% (P. Rhushi Prashad, Byregowda, & Gangavati, 2010) of its highest value and the effect of rainwater on the flat-plate collector provides cleaning for the flat-plate collector glazing.
Efficiency of Solar heat collector
The efficiency of solar water collectors depends on various properties such as resistance to UV radiation, good properties of thermal stability, and should be achieved at low costs. The desired temperature range for solar water heating systems is usually no greater than 60oC. The solar collectors used for solar water heating systems usually function with an efficiency between 40-50% (Shepherd D. , 2004). Figure 6 shows typical efficiency values for flat-plate collectors.
Flat plate and Evacuated tube collectors are affected by factors such as the construction of the collector, solar factors which include things such as radiation density and diffuse fraction, ambient conditions such as the weather and the operating conditions of the fluid system of the solar panels.
The valuable heat output is given by the equation:
Qu = mCp (To - Ti)
Where, Ti = Temperature of fluid entering the collector
To = Temperature of fluid at the collector outlet
m = Fluid flow rate through the collector
The thermal efficiency is then calculated as
The heat energy delivered by flat-plate collector is calculated by taking into account the heat energy absorbed by the flat-plate collector and subtracting this value from the heat that is lost to the environment.
There is a difference with the type of efficiency the flat-plate collector might have. It can be one of either two things that is, it can have linear efficiency characteristics or non linear efficiency characteristics (Gordon, 2001).
The optical efficiency of a flat-plate collector shows the amount of heat gained to the collector when the thermal losses are at zero (T=Ta), UL represents the linear function of the heat loss coefficient and UL = UL1 + UL2 ( T-Ta) is the dependant temperature of the heat loss coefficient.
The flat-plate collector operates at a maximum operating temperature known as the stagnation condition this is when there is no useful heat removal (Gordon, 2001).
As can be seen from figure 6, the efficiency of an unglazed solar collector is very high initially, however the thermal losses to the environment occur very rapidly and thus the efficiency declines as the temperature of the fluid becomes greater than that of the ambient temperature.
The efficiencies of the single, double-glazed and selective surface double glaze collectors are within a similar region between 50-70% (Fig 6) however, the single glazed surface appears to start off with a much higher efficiency than the other two glazed surfaces but like the un-glazed collector surfaces heat loss occurs quickly and thus the efficiency is dropped. The double glazed selective surface shows a similar efficiency to start of with to the single glazed surface and is able to maintain it for longer with minimal heat loss to surroundings/
Passive Solar Water Heating
Unlike an active solar water heating system that uses some form of mechanical or electrical pump or fan to circulate the fluid around the system, passive solar heating systems use natural means of convection to circulate fluid around through the solar collector to the storage tank.
There are two main types of passive solar heating systems known as thermosiphon solar hot water heating and batch system solar hot water heating.
Batch Solar Water Heating
The simplest type of solar hot water system available to anyone is known as the batch solar heating. This type of solar heating system consists solely of a water tank that is mounted on the roof of a building.
Solar batch water heating works by the tank absorbing solar radiation throughout the day. When one wants to use the hot water from the tank the water flows through a pipe system and is fed to the required destination inside the building. Batch solar water heating is a reliable, economical and simple method of heating water for hot climates such as South America, Africa and the Middle East but would be very impractical for consumers in places such as Europe. This is due to the fact that the tanks are generally not insulated and therefore lose a lot of the heat energy stored at night.
Aside from the basic outlay of a batch water heating system more efficient models have been developed and it consist of a black water tank contained within a glass covered insulated collector. This type of batch system is known as integrated collector storage (ICS) (Chiras, 2006) .
Integrated Collector Storage
The main reason for the development of the ICS is to reduce the thermal losses to the surroundings at night and to increase the intake of solar radiation. The most simplest design of a batch system as describe above is just a simple tank which is left to heat up and lost large amounts of heat at night. This design is not very ideal and does not allow it to compete very well with other solar water heating systems on the market. For these reasons the designs had to be improved to improve the performance. Figure 7 shows the mechanisms involved in the design of the ICS.
In an ICS system the hot water tank acts as the solar absorber. A typical material for the use of the storage tank is steel while the pipes are generally made out of copper. The ICS collector can be arranged in either a tank type configuration or in a tube type configuration.
Generally the tube type configuration provides improved performance compared to the tank type because there is a larger surface area for the solar radiation to strike. The tube configuration also provides a more aesthetically pleasing product which would be more favourable with potential customers.
The main component in the design of the ICS is the vessel. The vessel functions by absorbing solar insolation and transferring the thermal energy to its required end use point. The efficiency of such a system depends upon factors such as size, shape, materials, orientation and these all affect the amount of solar radiation the vessel is able to absorb.
The size and shape of the vessel affects the amount of solar radiation it can absorb. “The greater the exposed surface area to volume ratio the less time will be required for the insolation to heat up the water and store (Smith, Eames, & Norton, 2004)”.