ENGINE
1BLESSY JOY, 2PRIYA JOHN, 3ANJUMAN HABEEB, 4JEFFY MARIN JOSE
1,2,3,4
Department of EEE, Amal Jyothi College of Engineering, Kanjirapally, Kottayam, Kerala. E-mail: [email protected], [email protected], [email protected], [email protected]
Abstract—Solar energy is one of the renewable forms of energy being utilised for power generation, but it is not effectively
utilised. Solar powered stirling engines is a better option for effective utilisation of solar energy for electricity production. Earlier, solar energy was utilised by photovoltaic panels for small scale remote use applications. Now, solar thermal technologies especially concentrating solar power is getting attention. Dish Stirling System is one of the Concentrating Solar Power (CSP) technologies. At present, solar powered stirling system is not being utilised in many countries. Stirling engine is unique in the sense that it uses only two pistons for motoring action. It is a better solution for increasing demand of power. Unlike Internal Combustion Engines, it has high efficiency, and its operation is noiseless due to the absence of exhaust valves. The heat source used for the operation of the whole system can be of any burnable fuel such as straw, rice husk, wood and cheap combustibles. This way of generation ensures to be eco-friendly too i.e., this type of power generation yields no pollutants as by products. Molten salt heat storage system stores energy whenever power generation is not required. This is proven to be the most efficient way to convert solar energy to electric power. It is twice efficient and five times cost effective as compared to photovoltaic cells. Stirling engines have got a variety of applications in the field of space exploration, military purposes, heating and cooling purposes etc.
Keywords —Stirling Engine,Concentrating Solar Power,Molten Salt Storage System.
I.INTRODUCTION
One of the ways of meeting increasing demand of power is by producing power ourselves without depending on government. We can even sell excess of electricity if connected to the grid. This paper focuses on the self generation of electricity by making use of solar powered stirling generators [1].
We were depending on conventional fuels such as coal, diesel, nuclear for meeting increasing power demand. Coal, which is the major resource for power production will last hardly for another 155 years. So, we need to find another alternative fuel for production of power. The everlasting and clean source of energy production is from solar power. There are many different types of solar energy systems that will convert the solar resource into a useful form of solar energy conversion can be achieved mainly from solar thermal collectors and Photovoltaic collectors [2].
A. Overview of Solar Energy Conversion Systems A block diagram showing three of the most basic system types is shown as Figure 1.1.In the first diagram , the solar resource is captured and converted into heat which is then supplied to a demand for thermal energy (thermal load) such as house heating, hot water heating or heat for industrial processes. This type of system may or may not include thermal storage, and usually include an auxiliary source of energy so that the demand may be met during long periods with no sunshine [5].
If the demand (load) to be met is electricity (an electrical load) rather than heat, there are two common methods of converting solar energy into electricity. One method is by collecting solar energy
as heat and converting it into electricity using a typical power plant or engine; the other method is by using photovoltaic cells to convert solar energy directly into electricity. Both methods are shown schematically in Fig.1.
In general, if solar energy conversion systems are connected to a large electrical transmission grid, no storage or auxiliary energy supply is needed. If the solar energy conversion system is to be the only source of electricity, storage and auxiliary energy supply are usually both incorporated. If the thermal route is chosen, storage of heat rather than electricity may be used to extend the operating time of the system. Auxiliary energy may either be supplied as heat before the power conversion system, or as electricity after it. If the photovoltaic route is chosen, extra electricity may be stored, usually in storage batteries, thereby extending the operating time of the system. For auxiliary power, an external electricity source is the only choice for photovoltaic systems [2]. Solar energy can be converted to electricity mainly by two ways such as concentrating solar power using Solar Thermal Collectors and Photovoltaic Collectors. The solar concentrators used in concentrating solar power systems can often also be used to provide industrial process heating or cooling, such as in solar air-conditioning.
Concentrating technologies exist in four common forms, namely parabolic trough, enclosed trough; dish Stirling, concentrating linear Fresnel reflector, and solar power tower. Solar photovoltaic power generation has long been seen as a clean sustainable energy technology which draws upon the planets most plentiful and widely distributed renewable energy.
Fig.1: Solar Energy Conversion System [5]
II.LITERATURESURVEY
A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work or more specially, a closed-cycle regenerative heat engine with a permanently gaseous working fluid, where closed-cycle is defined as a thermodynamic system in which the working fluid is permanently contained within the system, and regenerative describes the use of a special type of internal heat exchanger and thermal store, known as the regenerator. It is the inclusion of a regenerator that differentiates the Stirling engine from other closed cycle hot air engines. This motor doesn’t have parts like stator or rotor but it has just a piston, cylinder a displacer (as shown in Fig.2) which produces the motoring action, which will be coupled with an alternator to produce power. The suns energy is focussed using Solar Beam Concentrator. One Solar Beam Concentrator can produce peak 10 kW/hour [3].
The Stirling engine was invented and patented by Robert Stirling in 1816.It followed earlier attempts at making an air engine but was probably the first to be put to practical use when in 1818 an engine built by Stirling was employed pumping water in a quarry. It is widely supposed that as well as saving fuel, the inventors were motivated to create a safer alternative to the steam engines of the time , whose boilers frequently exploded, causing many injuries and fatalities.
The need for Stirling engines to run at very high temperatures to maximize power and efficiency exposed limitations in the materials of the day, and the few engines that were built in those early years suffered unacceptably frequent failures.
Like the steam engine, the Stirling engine is traditionally classified as an external combustion engine, as all heat transfers to and from the working fluid take place through a solid boundary (heat
exchanger) thus isolating the combustion process and any contaminants it may produce from the working parts of the engine. This contrasts with an internal combustion engine where heat input is by combustion of a fuel within the body of the working fluid.
Fig. 2 : Stirling motor and its parts [1]
III. GENERATION OF ELECTRICITY
USINGSTIRLINGENGINE
A solar Stirling Engine (or Hot Air Engine) takes advantage of the fact that concentrated sunlight is a fantastic heat source, and as such can be used to generate electricity more efficiently than photovoltaic solar panels.
The Stirling Engine was developed in order to offer an alternative to the frequently explosive early steam engines. Basically a closed cylinder containing a piston and helium, nitrogen or hydrogen gas is heated at one end by concentrated sunlight, and cooled at the other end by air or water [6].
As the gas expands and cools with the movement of the piston, a generator can be driven to produce electricity. If the engine is run in reverse then it produces a cooling effect acting as a 'Stirling cooler'. With a Stirling engine combustion occurs outside the engine which made it much safer and less likely to explode. Stirling Engines did not catch on in the nineteenth century because of the costs of manufacture despite exceptional efficiency of almost 50% in some cases.
A. Operating Principle
Stirling engine works on the principle of Stirling cycle. The idealised Stirling cycle consists of four thermodynamic processes acting on the working fluid. This is shown in Fig.3 [4].
1. Isothermal Expansion. The expansion-space and associated heat exchanger are maintained at a constant high temperature, and the gas undergoes near-isothermal expansion absorbing heat from the hot source.
2. Constant-Volume (known as Iso-volumetric or isochoric) heat-removal. The gas is passed through
the regenerator, where it cools, transferring heat to the regenerator for use in the next cycle.
3. Isothermal Compression. The compression space and associated heat exchanger are maintained at a constant low temperature so the gas undergoes near isothermal compression rejecting heat to the cold sink.
4. Constant-Volume (known as Iso-volumetric or isochoric) heat-addition. The gas passes back through the regenerator where it recovers much of the heat transferred in 2, heating up on its way to the expansion space.
Fig.3: Stirling cycle [4]
When the gas is heated, because it is in a sealed chamber, the pressure rises and this then acts on the power piston to produce a power stroke. When the gas is cooled the pressure drops and this means that less work needs to be done by the piston to compress the gas on the return stroke, thus yielding a net power output. When one side of the piston is open to the atmosphere, the operation is slightly different. As the sealed volume of working gas comes in contact with the hot side, it expands, doing work on both the piston and on the atmosphere. When the working gas contacts the cold side, its pressure drops below atmospheric pressure and the atmosphere pushes on the piston and does work on the gas.
To summarize, the Stirling engine uses the temperature difference between its hot end and cold end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled and compressed, thus converting thermal energy into mechanical energy. The greater the temperature differences between the hot and cold sources, the greater the thermal efficiency.
B. Applications of stirling engines
Applications of the Stirling engine range from mechanical propulsion to heating and cooling to electrical generation systems. The Stirling cycle heat engine can also be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction (i.e. a heat pump, or refrigerator). 1) Military applications
Stirling engines can be used for auxiliary electrical production in order to provide the vital functions in the event of unavailability of the main source. Its silence of operation is a major asset in this application. Some military ships also use this
technology for corvettes or boats for mine detection or acoustic monitoring.
2) Spatial domain
Some satellites get energy through a Stirling engine. The efficiency is particularly high considering the great differences in temperature.
3) Cryogenic domain
The reversibility of the Stirling engine is used in order to produce cold in an industrial way. Its efficiency is then excellent. We provide mechanical energy to the engine. In fact, we transfer calories from the cold source the hot source, like in a domestic refrigerator. This mode of operation is so efficient that we use this type of installation to liquefy certain gas.
4) Acoustic Stirling heat engine
It converts heat into intense acoustic power which can be used directly in acoustic refrigerator pulse-tube refrigerators to provide heat-driven refrigeration with no moving parts, or to generate electricity via a linear alternator or other electro-acoustic power transducer. 5) Combined heat and power applications
In a combined heat and power (CHP) system, mechanical or electrical power is generated in the usual way, however, the waste heat given off by the engine is used to supply a secondary heating application. This can be virtually anything that uses low temperature heat. It is often a pre-existing energy use, such as commercial space heating, residential water heating, or an industrial process.
C. Advantages of Stirling Systems
Stirling engines can run directly on any available heat source, not just one produced by combustion, so they can run on heat from solar, geothermal, biological, nuclear sources or waste heat from industrial processes.
A continuous combustion process can be used to supply heat, so those emissions associated with the intermittent combustion processes of a reciprocating internal combustion engine can be reduced.
Some types of Stirling engines have the bearings and seals on the cool side of the engine, where they require less lubricant and last longer than equivalents on other reciprocating engine types.
The engine mechanisms are in some ways simpler than other reciprocating engine types. No valves are needed, and the burner system can be relatively simple.
Crude Stirling Engines can be made using common household materials.
A Stirling engine uses a single-phase working fluid which maintains an internal pressure close to the design pressure, and thus for a properly designed system the risk of explosion is low. In comparison, a steam engine uses a two phase gas/liquid working fluid, so a faulty overpressure relief valve can cause an explosion.
They start easily and run more efficiently in cold weather, in contrast to the internal combustion which starts quickly in warm weather, but not in cold weather.
A Stirling engine used for pumping water can be configured so that the water cools the compression space. This is most effective when pumping cold water.
They are extremely flexible. They can be used as CHP (combined heat and power) in the winter and as coolers in summer.
Waste heat is easily harvested compared to waste heat from an internal combustion engine making Stirling engines useful for dual-output heat and power systems.
D. Practical difficulties in Stirling Systems
Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This increases the size of the radiators, which can make packaging difficult. Along with materials cost, this has been one of the factors limiting the adoption of Stirling engines as automotive prime movers.
Power output of a Stirling tends to be constant and to adjust it can sometimes require careful design and additional mechanisms. Typically, changes in output are achieved by varying the displacement of the engine, or by changing the quantity of working fluid, or by altering the piston or displacer phase angle, or in some cases simply by altering the engine load. This property is less of a drawback in hybrid electric propulsion or "base load" utility generation where constant power output is actually desirable.
The gas used should have a low heat capacity, so that a given amount of transferred heat leads to a large increase in pressure. Considering this issue, helium would be the best gas because of its very low heat capacity. Air is a viable working fluid, but the oxygen in a highly pressurized air engine can cause fatal accidents caused by lubricating oil explosions.
IV.MOLTENSALTHEATSTORAGESYSTEM
Molten salt can be employed as a thermal energy storage method to retain thermal energy collected by a solar tower or solar trough so that it can be used to generate electricity in bad weather or at night. The system is predicted to have an annual efficiency of 99%, a reference to the energy retained by storing heat before turning it into electricity, versus converting heat directly into electricity. The molten salt mixtures vary. The most extended mixture contains sodium nitrate, potassium nitrate and calcium nitrate. It is non flammable and nontoxic, and has already been used in the chemical and metals
industries as a heat-transport fluid, so experience with such systems exists in non-solar applications.
The salt melts at 131℃ (268 ° F). It is kept liquid at 288 ℃ (550 ° F) in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 ℃
(1,051° F ). It is then sent to a hot storage tank. This is so well insulated that the thermal energy can be usefully stored for up to a week.
When electricity is needed, the hot salt is pumped to a conventional steam-generator to produce superheated steam for a turbine/generator as used in any conventional coal, oil or nuclear power plant.
CONCLUSION
This paper discusses about the importance of Stirling engines for producing electricity from solar power. This invention could really change the world because the current method of delivering electricity is extremely insufficient. A new power plant will typically burn natural gas, but there are a lot of losses between the generation and consumers. It would be much more efficient to generate the power at consumer side by making use of renewable sources. Thus we can generate power on our own and atleast compensate our additional needs thereby sharing the power with our neighbours and leaving way for the future generations to make use of the resources because energy saved is energy produced. This is an eco-friendly power production and it can be said that Stirling engine is a "extra green, extra quiet, extraordinary."
This paper also discusses about the solar heat salt storage system, mainly molten salt storage system. This storage system provides lower salt requirement, higher steam cycle efficiency, better compatibility with air cooling, improved winter performance, and simplified piping schemes. Molten-salt power towers and concentrating solar plants with storage in general, are well-placed to provide dispatchable power for 100% renewable grids in sun-belt countries.
Future advances in molten-salt power tower technology include improvements to the thermal properties of molten salts and the development of storage solutions in a single tank.
ACKNOWLEDGMENT
First of all, authors would like to thank God almighty, for best owing his blessings in this endeavour. The authors also acknowledge the technical support from all those who guided and helped during the preparation of this report.
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