ENN552 Solar Thermal Systems

1.0 Introduction:

About 87% of the World energy demand is supplied by Fossil fuels only less than 13% of renewable sources of energy is used to supply the energy demand in world wide. Energy generation by fossil fuels creates a very negative impact on environmental health as the emission of harmful gases occurs due to burning these fuels. The production of electricity from the renewable sources are the most environmental friendly method as the renewable sources of energy are self-replenishing and inexhaustible. The Solar energy is a renewable source of energy, which is collected from the sunlight. There are many methods to generate electricity from the solar energy such as PV cells and Thermal collectors. The PV cells utilizes the photovoltaic technology to convert the sunlight into electrical energy directly with the help of semiconductors (Mehrara, M. 2007).

The thermal collectors collects the thermal energy from the sunlight and utilizes the thermal energy to generate steam and to run a turbine which is coupled with electric motors and generates electricity. Some of the solar thermal collectors that are being used are parabolic trough collector, Fresnel lens collector, Heliostat field central receiver system, etc. The flat plate and the parabolic collectors are used for small and medium power generation capacities. The heliostat thermal collectors are applicable in higher power generation plants. (Boyle, G. 2004).

2.0 Power generation using Solar Thermal Collectors:

The Solar thermal collectors collects the heat energy from the sunlight and utilizes it to generate electricity with the help of various arrangements such as the reflective mirrors, Heat exchanger, turbine, Heat transfer Fluid, generator, etc. (the schematic of a simple solar thermal collector is shown in the Figure2.) The heat transfer fluid are allowed to flow through the tubes which are placed on the reflective mirrors, when the sunlight hits the reflective mirrors the heat energy is concentrated into the tubes which in turn supplies heat energy to the HTF. Then with the help of a heat exchanger the heat transfer takes place. Inside the heat exchanger the generation of steam is done. The generated steam is allowed to pass through the turbine. The mechanical energy obtained from turbine is converted into electrical energy with help of generators.

2.1 Heat Transfer Fluids:

The Heat Transfer Fluid is used to transfer the heat energy from the collector to the heat exchanger. Some of the variables that are considered during the selection of the HTF are its Coefficient of expansion, Viscosity, Thermal storage capacity, Freezing and Boiling points. The selection of the HTF is greatly influenced by the environmental conditions in which the power plant is about to work. For example if a plant is placed in a hot dessert area then the HTF should have high boiling point, proper viscosity and a proper thermal storage capacity.(Rached, W. 2011).

Some of the most important types of Heat Transfer Fluids are:

Air

Air is an excellent, cheap, and affordable HTF. Air has anti freezing property and it also does not boil so it is applicable both in extremely cold and hot conditions. On the negative side it has extremely low heat carrying capacity, also there will be leakage problem in ducts. (Reddy, J. N.  2014).

Water

Water is an excellent form of HTF as it has very high heat carrying capacity and it is cheap and affordable. On the other hand it has very high freeing point and low boiling point. Water is applicable in case of low to medium temperature range operation.(Reddy, J. N.  2014).

Oil

Oils generally have high viscosity but the heat carrying capacity of oil is lower than that of the water. They also have high specific weight of gravity. There are generally 3 types of oils such as synthetic, semi synthetic and normal.(Reddy, J. N.  2014).

Molten Salt:

The molten salts are found to have greater heat carrying capacity. They are applicable in case of super heating type of solar power generators. It words under extremely high temperatures.(Reddy, J. N.  2014).

2.2 Types of solar thermal Collectors are:

Parabolic Trough

The parabolic trough utilizes a collector with parabolic cross section. The heat is concentrated at the focal point of parabola. (Herrmann, U., Kelly, B., & Price, H. 2004).

It focusing effect of Fresnel lens is utilized. In this type of collector the radiation from sun is focused to absorber from top not in bottom as in case of the parabolic collectors.

Large numbers of mirrors are arranged as a heliostat, the heliostat is nothing but a mirror which can track the sun and face on its direction always. (Noone, C. J et.el., 2012).

2.3 Comparison of various types of solar thermal collectors:

From the comparison table we can see that the parabolic trough solar thermal collectors are used in case of low power generation power plants. The Linear frensel collector is applicable in case of the medium power generation range, also the Fresnel is cost wise efficient and it is reliable in an acceptable level. The Heliostat field is used in case of power generation requirement in a range of 30 to 200MW and it is costly.

2.4 Working of a solar thermal power plant:

In general a solar thermal power plant consists of the following major parts:

• Solar Thermal Collector
• Turbine
• Generator
• Condenser
• Cooling tower

From the image we can see that the water is pumped from the water source to the solar thermal collector, the water gets heated as it absorbs the heat that are collected from the sun with the help of solar thermal collectors. The water converts into steam once it reached its phase change point, then the heated steam is allowed to pass through the turbine, a turbine is a mechanical arrangement of number of blades connected to a single shaft in order to convert the kinetic energy of the steam into the mechanical energy. As the steam passes through the blades of the turbine it turns the shaft, which is coupled to the generator. A generator generates electrical energy from mechanical input. The electrical energy that is obtained from the generator is then passed to the storage/ usage through the transmission lines. The steam after passing through the turbine is allowed to pass through the condenser which cools down the steam into liquid water with the help of cooling tower by losing heat to the surroundings.

The whole system works on the basis of rankine cycle also known as vapour powered cycle. If the working fluid in a cycle is a phase change material which converts the kinetic energy of the working fluid into useful work then the system is said to be following a ranking cycle. A rankine cycle is a heat engine which is used to measure the operational condition of a power plant. A rankine cycle consists of 4 processes such as pumping the water, heating or converting the water into steam, generating mechanical output, and condensing steam into water. 

Efficiency of a rankine cycle:

The thermal efficiency of the Rankine cycle is given by,

The efficiency of the turbine is given in terms of isentropic efficiency, h_T as:

Efficiency of the pump is given as:

The performance of the rankine cycle can be increased by many ways such as:

• Lowering the pressure of the condenser.
• Super heating.
• Increasing the pressure of the boiler.

3.0 Selection of appropriateSolar thermal technology

Let us consider a solar thermal power plant is needed to be built in Darwin, Australia.

So the requirements of the power plant are:

• 25MW capacity
• Guaranteed 10 hrs power output
• Max 15% auxiliary fuel contribution to the output.

 Darwin’s weather data shows that in a month atleast an average of 13 hours of daylight is available in Darwin. (Lucas, C. 2010).

The optimal system that is suitable for this requirements and the weather is the heliostat system. The heliostat system is very efficient and can give the guaranteed energy output of 10hrs per day. Also as the heliostat system is chosen because of its operation range and the power generation capacity. As the heliostat is equipped with sun tracking system it is highly efficient. (Sanchez, M., & Romero, M. 2006).

Formulae for Calculations

The Fraction of ground at which the heliostats are covered is calculated as,

Now,

is calculated as:

Where

Energy absorbed by the receiver:

Where,

The efficiency of solar thermal power plant:

The efficiency of the overall power plant depends upon various factors such as incident solar radiation, amount of daylight, hours of daylight, solar thermal collector and the storage efficiency. The solar incident radiation falling on the solar collector is measured as:

The amount of solar power received per unit area:

We is the solar insolation

A_c =

Now the overall efficiency of the thermal collector is given as:

P: parameters of inlet fluids

Ti: Temperature of inlet fluid

Ta: Ambient air temp

I: Solar radiation falling on collectors.

4.0 Energy Storage System

In order to bridge the gap between the energy demand and supply we need to have an energy storage system which can store the excess amount of heat energy available at day time and store it for the usage during night times. Also the storage device acts as a backup or auxiliary energy supply system when the climate is dully or when the energy demand is higher than the supply. The power plant that we have designed is of 25 Mega Watt capacity. So, the energy storage device should also be capable of storing such a huge amount of energy.(Zalba, B et.el., 2003). The storage of thermal energy is difficult still it is better than using batteries to store the energy. Electric Batteriesare more expensive way of storing energy as the battery life is short and there are many losses associated with it. So the best option is to go for Thermal batteries, they  are the best way to store the available thermal energy, the energy loss is less as the energy is stored in its own form and not converted from one form to another as in case of electric batteries (Zalba, B et.el.,2003).

A Thermal Battery is an energy storing device which can store and release the thermal energy. A thermal battery must be well insulated from the surrounding so that there will be no energy loss. The energy is generally stored in two forms the first one is sensible heat and the second one is the Latent heat, the latent heat energy storage is possible in case of Phase Change Materials (PCM) only. The PCMs having exceptional property of changing from one phase to another with the change in temperature, during these phase transition these PCMs absorbs a large amount of energy in form of latent heat.(Zalba, B et.el., 2003).  

Molten Salt Energy Storage System:

Molten Salt Energy Storage device (MSESD) is a system which provides exceptional heat storage capacity and it suits best for the power plant of large capacities, in our system 25 MW. At the atmospheric pressure the molten salt is liquid and it can be operated at very high temperatures as it is nontoxic and non-flammable. The molten salt that is encapsulated inside the thermal battery stores the thermal energy, there are two processes that takes place in this process the charging and the discharging processes. The charging process takes place by allowing the excess heat to flow through the battery via tubes. The HTF carries the heat and discharges the heat to the molten salt. Then during the time of energy demand the discharging process takes place, by allowing the cold HTF to pass through the battery which when heated once passed through the battery. The discharged heat is used to make useful work. The molten salt consists of Salpetere, 60 percentage sodium nitrate and 40 percentage of potassium nitrate. Comparing to other storage methods this method is cost effective and it gives life greater than 30 years. 

Formulae used:

Calculation of Amount of energy that can be stored in a thermal battery:

Total Amount of heat energy that can be stored,  Q = Q₁ + ΔH +Q₂

Where,

Amount of Sensible heat transfer, Q₁ = M_salt * C_p(salt)*(T_pi1-T_fi1)         

Amount of Latent heat transfer, Q₂ = M_salt * C_p(salt)*(T_pi2-T_fi2)

Overall heat transfer,  Q = U*A*(ΔT)   

Enthalpy, ΔH = (M_salt* h_f) ÷ t  

Calculation of amount of energy stored in the given Storage system:

The temperatures are measured from the storage system as:

T_pi1 = 500⁰ C

T_pi2 = 300⁰ C

T_fi1 = 320⁰ C

T_fi2 = 430⁰ C

C_p(salt)  = 1.53 J/(g K)

Now applying the above data in the formulae and calculating we get,

Now overall heat energy that is stored is calculated as:

From this it is clear that the overall storage capacity meets the requirements.

References:

Boyle, G. (2004). Renewable energy. Renewable Energy, by Edited by Godfrey Boyle, pp. 456. Oxford University Press, May 2004. ISBN-10: 0199261784. ISBN-13: 9780199261789, 456.

Herrmann, U., Kelly, B., & Price, H. (2004). Two-tank molten salt storage for parabolic trough solar power plants. Energy, 29(5-6), 883-893.

Lucas, C. (2010). On developing a historical fire weather data-set for Australia. Australian Meteorological and Oceanographic Journal, 60(1), 1.

Mehrara, M. (2007). Energy consumption and economic growth: the case of oil exporting countries. Energy policy, 35(5), 2939-2945.

Noone, C. J., Torrilhon, M., & Mitsos, A. (2012). Heliostat field optimization: A new computationally efficient model and biomimetic layout. Solar Energy, 86(2), 792-803.

Rached, W. (2011). U.S. Patent Application No. 13/122,606.

Reddy, J. N. (2014). An Introduction to Nonlinear Finite Element Analysis: with applications to heat transfer, fluid mechanics, and solid mechanics. OUP Oxford.

Sanchez, M., & Romero, M. (2006). Methodology for generation of heliostat field layout in central receiver systems based on yearly normalized energy surfaces. Solar Energy, 80(7), 861-874.

Zalba, B., Mar?n, J. M., Cabeza, L. F., & Mehling, H. (2003). Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied thermal