Renewable energy and its various applications are widespread among the modern society, solar water heating systems are quite popular amid them. Modern science and technology influenced abundantly in the development of a variety of solar systems and still developing more effective and efficient ones. Solar radiation from the sun is received by the absorber in a solar system and then converted into different energy forms. There are different analyzing methodologies like energy and exergy analysis to produce efficient systems and to reduce the system cost.
This paper implicates the exergy analysis of a solar water heating system to figure out the losses during the heating procedure by employing the second law of thermodynamics.Solar irradiance, ambient temperature, Fluid temperature and mass flow rate are investigated for carrying out the calculations.
In this project, exergy analysis is performed in a closed loop solar water heating system consisting three major parts; a flat plate solar collector, heat exchanger and a pump for circulating the fluid. The main objective of this study is to assess a domestic solar water heating system using exergy analysis and thereby understanding how exergy will affect the overall efficiency of the system.
The solar power has been used for several purposes from ancient days; for lighting torches, burning ants and so on. The utilization of solar power went on improving as the years passed on and then the scenario has changed it to the most reliable renewable energy resource. When it comes to alternative energy resource in terms of water heating, solar is considered to be the best option among all, because of the minimal environmental impact and cost efficiency. In general solar mode of water, heating will not be an economical method straight away after the installation it will take some time to be financially favorable form a customer’s point of view. By using Solar powered water heating system of 100 liters, annually 1.5 tonnes of carbon dioxide emission can be reduced.
In a solar water heater, the heat from the sunlight is collected using a solar collector, which is used to heat up the liquid flowing through the collectors. This heat is used for the heating of water in the heat exchanger. The type of the system differs depending on the climate and latitude of the place, along with that the cost of the system also changes.
Though the sector is still on its development stage as the cost and the technology has to be still improved considering its efficiency, cost, and overall performance.
In this contemporary world, the harmful impacts due to the over usage of fossil fuels are alarming. The effects due to this are degrading air quality due to increase in air pollution, climate change, rising of temperature, reduction in the annual rainfall, sinking levels of fossil fuel reserves and the like. Thus the concept of clean energy is gaining attention these days and the most of the energy demands will be satisfied through sustainable energy in the upcoming era.
Solar Water Heater
We are availing a huge amount of solar energy from the evolution of our world at absolutely no cost. It is estimated that one square meter of earth’s surface is receiving 1KW of energy from direct sun light. The modern science has derived different techniques to conveniently use this energy either in the form of heat or electricity. Solar water heating systems are popular among these.
A basic solar water heating system contains a solar collector and a storage tank. Water passing through the tubes in collector got heated up and delivered to the storage tank. In a sunny day of summer, solar water heater can heat the water from 60 to 80 degrees Celsius.
There are two major classifications of the solar heating systems
1. Active Systems
2. Passive Systems
Active systems use electrical components such as pumps, valves and controllers for circulating the water or heat transferring fluid. These systems are comparatively efficient as well as expensive.
Active systems are further classified as Active Open Loop and Closed Loop systems
1. Active Open Loop System
Open loop systems are simple in design and operating cost is less compared to the closed loop systems. Potable water from the storage circulate through the collector to heat up with the help of a pump. This model is suitable where the temperature is not so cold as the water in the circulating tubes freeze in extreme cold climates. It can be installed in occasionally freezing climates with freezing protection where the hot water from the tank recirculate through the collector when it starts to freeze but periodic usage of freezing protection cause heat and electricity ravage.
2. Active Closed Loop System
An intermediary anti freezing fluid (eg: Propylene Glycol) is employed to exchange the heat from collector to the water in the storage tank. Closed loop systems are suitable for all climates, even in extreme cold temperatures as they offer better freeze protection. These systems usually have a heat exchanger to transfer heat from fluid to water. However, the operating and maintenance cost are a bit higher than open loop systems.
Passive systems are simple, cheaper, reliable and long lasting as they contain no electrical elements. These system does not use pump; the water circulates automatically when heated because of the pressure difference. It is suggested for the households with evening hot water needs because most of the heat collected lose overnight and will not be efficient for the morning needs. Passive systems are preferable for the areas where temperature rarely go below freezing.
Other commonly used solar water heaters are below.
Batch heaters are a kind of simple passive systems, which involves single or multiple storage tanks. The tank is kept in an insulated box with a transparent surface facing sun and act as a collector. By doing so the heat loss can be reduced. The tank absorbs the solar energy and warm the water in it. When the hot water is drained from the outlet at top of the tank, cold water will be filled through the inlet at the bottom. The system must have freezing protection in climates where freezing happens. As the system not using any pumps and controllers, this is the cheapest among other systems and least effective too.
Solar ponds are huge man-made salt water body to store solar energy to provide heat. The principle behind the solar pond is very simple as we know that hot air or water is light in density and rise to the top. Normal fresh water ponds lose the heat to atmosphere when heated up. The salt in the solar ponds makes the water in bottom layer very dense and prevent the water from rising when heated, as a result the water underneath stays hot. A solar pond has three layers, the bottom layer known as Storage Zone where the heat is stored. The middle layer is known as Gradient Zone which inhibit the convection and act as an insulator for storage zone. The water remains in atmospheric temperature in top layer, that is Surface Zone. When the hot water is pumped out for consumption, surface layer will be filled by fresh water. There are many other applications for the solar ponds rather than heating like electricity production and water purification etc.
Evacuated Tube Collector
An evacuated tube collector, also called as twin tube collector consist of a thick transparent outer glass which is high temperature resistive and a thin inner glass which has a coating to absorb sun light. Air between both the layers are evacuated and the vacuum will act as an insulator to prevent all kind of heat losses therefor attaining excellent efficiency. A heat pipe made up of copper and filled with fluids fixed inside the inner tube which deliver the heat from the collector to water in storage tank through convection. When the temperature rises fluid becomes vapor and this hot vapor exchange the heat and after condensing it returns to heat pipe as liquid. This cycle will continue until the last sunlight of the day. Hot water will be available over the night as the tank is insulated and capable of keeping it for longer period of times. Evacuated tube collectors are good substitutes for flat plate collectors especially in cold and cloudy climates but a little expensive.
Anti-freezing liquid (Heat transfer liquid)
Heat is collected carried and transferred using a liquid such as water or any kind of liquid which can act as a carrier. The fluid used should be selected under the consideration of some factors like freezing point, boiling point, viscosity and the like. The selection of the liquid will be also influenced by the climate condition of the place to be installed.
Property Propylene Glycol
Solar thermal collector
There are mainly three types of solar collectors used in a domestic hot water system:
Flat plate collectors
Integral collector-storage systems or batch systems
Flat plate collectors:
Glazed collector system
The flat plate collectors are designed with a dark flat plate which acts as an absorber of solar energy and this heat is trapped inside the setup by a transparent cover, which reduces the loss of heat along with a heat insulating backing and there are tubes in between which carries the liquid.
It is similar to the glazed collector system the difference is the absence of the transparent cover. Instead of that, the material used makes the heat absorbed. It is simple and inexpensive due to its design significance. These collectors are mainly used for swimming pool heating. But it’s less efficient when the atmospheric temperature gets cooler.
It is used to transfer the heat carried by the carrier to the water in domestic solar water heating system. Fluid such as water, oil, air or some type of anti-freezing liquids is generally used in heat exchangers. In cold climatic places, antifreezing liquids are used as a carrier of heat in heat exchangers in both water heating as well as space heating system. The liquid carrying heat will pass through heat exchanger chamber in a tube and transmit heat to the water without mixing with it. The tube in which the heat carrying liquid flow will be a double layered one, which is to prevent the mixing of liquid and water in case of any leakages because, antifreezing liquids are normally toxic in nature.
The essential basic necessity of using a pump is to push enough warmth exchange liquid through your sun oriented authorities to effectively evacuate the warmth that the sun is saving in them. Too little stream of liquid or excess of stream of liquid causes less productively and decreases efficiency
The pump is used in an active type domestic solar water heater system for circulating the liquid heat-carrying liquid in the system. It takes the anti-freezing liquid from the heat exchanger after the heat transfer and pumps it to the solar collector for continuing the circulation and collecting heat from the collectors.
Energy and Exergy :
Studies conducted on solar water on the First Law of Thermodynamics it tells us that energy as a conserved quantity; “Energy can neither be created nor destroyed it can only be transformed from one form to another”. According to Second law of thermodynamics a processes occur in a certain direction only and energy has quality and quantity. So, it is essential to evaluate solar water heaters from the of the Second law of thermodynamics point of view because it is the quality of energy that is significant not the quantity of energy.
Exergy Analysis :
Exergy analysis is done with the motto of providing some methods to find the possibilities of improvements in the system. In Exergy analysis of solar water heater, the exergy efficiency of collector, heat exchanger and the pump is calculated. Exergy analysis has been widely used for the optimisation of losses in the energy systems. Exergy is the expression for loss of available energy due to the creation of entropy and its irreversibility. The exergy loss in a system is determined by the temperature of the surroundings by the increase in entropy. Exergy is also the amount of maximum work that can be extracted from the system by the interactions of the surroundings
Location Fez (Moracco)
Solar radiation = 230 kwh/m2
Output temperature of the collector = 80oC
Input temperature of the collector = 44oC
Tank top temperature =69.3oC
Tank bottom temperature =44oC
Ambient temperature = 27o
To find the mass flow rate
Specific heat of propylene glycol, Cp =3694J/KgC
Density of propylene glycol, ?=1023.61Kg/m3
Height between pump and collector =5m
Input pressure in pump, p = 1 bar
Enthalpy, H1 = 417.36 KJ/Kg
Entropy, S1 = 1.3026 KJ/Kg
Output pressure in pump, p=1.5bar
Entropy, S2 = 1.4336 KJ/Kg
Specific volume at 1 bar, vf1 = 1.0432
Specific volume at 1.5 bar, vf2 = 1.0528
To find the output water temperature
Outer diameter of the pipe, d0 = 9.525mm
Inner diameter of the pipe, di = 7.035mm
Length of the pipe = 15m
Efficiency of the thermal collector:
? = ?o-a1 – a2
Mass flow rate of the collector
Energy Efficiency * Solar radiation = M*Cp*?T
M = 1.434 Kg/s
P1 = 1 bar (Atmospheric pressure)
P2 = ?gh
P2 = 1.5 bar
To find pump work
= 26.58 KJ
Exergy of thermal collector
To find the output temperature of the heat exchanger
M*Cp*?T = M*Cp*?T
Tx = 41.22oC
Exergy of heat exchanger
Pen(propylene glycol) = M*Cp*?T
Pen(propylene glycol) = Pen(water) = Pen
Pex(propylene glycol) = (1-)*Pen
?Px= Pex(propylene glycol)- Pex(water)