In this laboratory experiment, the efficiency curve of evacuated tubes solar collector is investigated. The experiment was held in an indoor environment The system is composed by a ten evacuated tube solar collector, a water tank to store the heat, a pump to circulate the fluid and a computer to record temperatures during a range of one hour. A maximum efficiency up to 44% has been registered. Critical explanations of the efficiency curve and its characteristics are discussed. Factors influencing the operation of the collector are listed and commented and a numerical study of the instantaneous efficiency is presented.
Subject:
With the limited supply of fossil fuels energy sources, the Alternative Energy methods to heat a house or to heat domestic water are diffusing more and more in the market. In fact, using natural resources, as free sources of energy, can be one method to reduce the demand of fossil fuels and to delay their unavoidable end. Solar energy is one method that can be used with good results in a big scale and in a small scale as well. The sun today regularly heats up buildings and domestic water, trough a heat collector and a storage unit.
Solar collectors are a type of solar thermal system. These systems use heat from the sun as an energy source. Solar collectors use sunlight to heat a fluid (for example water). The principal scope of a solar collector is to provide hot water for house’s everyday use and eventually for space heating. Solar collectors in homes typically provide water that is between 40 degrees C up to more than 80 degrees C.
The most common use of solar collectors in Europe is for Domestic Hot Water and house heating, while in America is principally used for heating swimming pools (especially in United State). Nowadays the price of solar collectors is more affordable than in the past and the performance of the collector has increased quite a lot. Technology can be considered quite stable even thought the research could still improve this system. All these factors make the Solar Collector an affordable technology widely used in different countries.
Solar collector scheme
| Picture 1: Main components of a Evacuated Tube Solar Collector system |
Solar collectors may provide about 70% of DHW, and up to 30% of home heat, depending on location, climate, latitude, season, hot water usage rates, and size of the system. Because in most of the cases solar collectors cannot fully provide in real time the required hot water and space heating for the house (especially at night), it is necessary to include one or more backup units (water heaters or boilers) in order to meet the demand.
Evacuated Tube Solar Collector system is today one of the most efficient way to produce DHW and house heating harnessing the solar radiation. In this experiment we used this typology of collector that represent the best (in terms of efficiency) Solar Thermal technology solution for DHW today available. Evacuated tube collectors are constructed of a number of glass tubes. Each tube is made of double layer annealed glass. The in-between space hosts the vacuum that is created during the manufacturing process. The Vacuum inside tubes allows reaching a good insulation and reduces heat losses. An absorber plate is than located inside the inner tube. Because of the insulation, high temperatures are achieved at absorber plates.
Objective:
The objective of this experiment is to investigate how a solar thermal collector is working and how to calculate its efficiency curve using a series of records taken during the experiment (see appendix: TABLE 1). Obviously the experiment have been held in a indoor controlled environment (laboratory at Energy Learning Unit, University of Nottingham) where natural light have been simulated using tungsten halogen lights. The Ambient temperature is considered the one of the laboratory. These approximations, together with other factors (like for example uncontrolled heat losses from the Water Tank) allow us in any case to understand the system, but we cannot consider the reliability of numeric values recorded in the lab to calculate the real efficiency of the collector. In fact, if we compare these data with the real values we could get in a real case, we may experience a big discrepancy. In a real case the system is optimized to reduce heat losses and maybe the sun is not shining with the same intensity we simulated in the laboratory.
Theory background:
As we said above, Evacuated Tube Solar Collectors use the light from the sun to heat a liquid that usually is water passing trough pipes. These hot water pipes then store the water in a solar hot water tank (like a hot water heater). The image below (Picture 2) shows the process that takes place within an Evacuated Tube.
The efficiency of a collector is the value that best describe the capacity of the collector to convert solar energy into thermal energy. Efficiency can vary depending on the solar radiation, outside temperature, and ambient temperature. Efficiency is calculated from the following equation. The efficiency is defined as:
Solar collector Evacuated Tube diagram section
| Picture 2: Mechanism that regulates the heat absorption in aEvacuated Tube Solar Collector |
Efficiency = What you get out/What you put in
With this equation, it is easy to see that output will be low if you have a low sunshine level input.
The instantaneous efficiency describes the ability of the system to convert the solar energy into heat. It is calculated through this formula:
Formula Efficiency
This is the first calculation we are going to consider in order to analyse the behaviour of the system and in order to consider only correct values when calculating the overall efficiency. It is necessary to remember that the system has a first phase of warm up followed by a regime phase. We are interested only in the regime phase, because some data collected in the warm up phase me be inaccurate.
Balancing the solar energy in a solar collector we get the following result: Q tot = Q gain+ Q loss
Heat Balance
| Picture 3: Solar radiation balance into solar collectors |
Heat Losses are related mainly to radiation and convection as shown in Picture 3. Moreover because of the properties of the components of the collector (glass, plate, insulation) we can experience reflection losses, convection and radiation losses.
In fact, the glass cover behaves nearly as a black body for long-wave radiation. It let the short wave radiation from the sun to come inside the tube collector, and traps inside the tube all reflected long wave radiations coming from the plate. In this way we should take into account all these value in order to quantify the real efficiency of the solar collector.
Heat Losses and Heat Gains
Therefore the efficiency could be represented by the following equation:
The previous equation for the efficiency could be expressed by:
Because of the coefficient of transmittance “t “of the glass and because of “ap“ (factor of absorption of the plate) the previous equation is formulated by Hottel-Willier in the following way:
Where“niopt” is called “optical efficiency” and it is indicated as “t +alphap”
Where “Tinitial”is the temperature entering in the plate at the initial stage (the meaning is the same as Tplate)
This is the formula we are going to use in order to analyse the efficiency plotting a curve into a graph. This curve is actually a LINE (y=bx+c) and is called Efficiency Curve. It allows us to understand the performance of the collector quickly and easily (Picture 5)
Curve of the efficiency
| Picture 4: Schematic curve of the efficiency |
Stagnation temperature is the temperature at a stagnation point in a fluid flow. At stagnation point the speed of the fluid is zero and all of the kinetic energy has been converted to internal energy and is added to the local static enthalpy.(Source: www.wikipedia.com)
The stagnation temperature is extremely important because if the system reaches that temperature the solar heat is not converted into thermal useful heat, but in heat losses. It means that the system is not more working and the efficiency is zero, even thought the temperature is high.
The following values are known:
In order to use the IS units, it is necessary first to convert some value:
Initial steps into the experiment:
The experiment consists in a series of processes that are listed as follows:
Laboratory took at least one hour to be completed. When the laboratory started, the first process was just concluded (the pump was turned on). The sensors connected with the computer were collecting the numeric values of temperatures. The start up phase of the solar collector was just running when we began to check every part of the system.
After about one hour (63 records of temperatures for 63 minutes) we were able to save the values in a excel file and to stop the system. It was absolutely clear that the system was still increasing the temperature in the inlet and in the outlet too. But according to the instructions the experiment was concluded after one hour system’s running.
Equipment:
| Picture 5: Main components of the analised system |
As we can see in the picture above, there are 6 main elements:
Light source: six tungsten lamps are simulating solarirradiance up to 1000 W/m2
Solar collectors: ten evacuated tube solar collectors. “Fins’ area” is globally 1m2.
Manifolds: this is a crucial part of the system. It is the place where water receives heat energy by collectors, and it should be isolated properly in order to improve the efficiency of the system.
Pump and sensors: Here are located in the same position. But they are two different elements of the whole system. Pump moves the liquid (water) in the circuit, while sensors are registering temperatures and sending back to the computer all the records.
Water tank: this is where heat is stored. Water contained inside the tank circulates in the circuit reaching the manifold where it is heated.
Computer: this is the “supervisor” of temperatures. A series of three thermometers register all the changes in the temperatures. the sensor switchboard receives this value and send them directly to the computer, where a software records the numeric value.
The following pictures show some elements of the system:
| Picture 8: Pump and compressor |
| Picture 7: Switchboard |
| Picture 6: Thermally isolated manifold location |
In the first picture we can see the connection between solar collectors and manifolds. The orange colored material visible in the junctions between the solar collectors and the grey box of the manifolds represent the insulation used to prevent heat losses.
In the second picture we can see the sensors’ switchboard. As we can see there are three couple of cables going inside the device. Those cable are connected to the three thermometers measuring Tambient, Tin, Tout.
In the third picture we can see the compressor (red) and the small pump (green) used to drive the water in the circuit. It is moreover visible the sensors switchboard previously mentioned.
Here are a series of pictures explaining some of the elements of which the evacuated solar collector is made out:
Top left picture shows three main elements that constitute the evacuated collector (from the top: evacuated tube, heat coupling metal, heat pipe). The right top picture shows the last part of the collector: the absorber. It is colored with a specific painting that is called “absorptive coating”. The last two pictures at the bottom show the evacuated tube by two different points of view.
Here at the left side we can see the bottom part of the heat pipe, while at the right side we can see the top element of it. This bulb is responsible for the heat transfer with the manifold. Inside this bulb the heat liquid vector condenses releasing heat to the water passing into the manifold.
Diagram of the system:
This is the diagram of the system:
In this simple diagram we can identify all the main elements previously mentioned (except for the computer that is connected to the controller here represented in green)
Diagram of the Heat Pipe:
 Picture 15 |
 Picture 16 |
The crucial point of the system is the Heat Pipe.The heat pipe is an evaporating-condensing device for rapid heat transfer. The latent heat of vaporization is transferred by means of evaporating a water-based liquid in the solar heat inlet region, and condensing its vapor in the water tank. The heat source is the absorber plate that is continuously connected to the heat pipe. The condenser (in our case bulb in Picture 14) is in close contact with a manifold, which works as a heat exchange. In addition, the heat pipe has a diode function. The heat transfer is always in one direction. The heat goes from the absorber to the manifold and from the collector to water storage tank and never the reverse.
These are the results of the analysis of the temperature:
| Graph 1 |
The following graph shows the plot of the instantaneous efficiency over time:
| Graph 2 |
Here is the efficiency over DT/I averaged line:
| Graph 3 |
Explanations of the results:
Each graph shows a series of characteristic of the collector and of the experiment. In Graph 1 we can se clearly the warm up phase of the collector. Temperatures at the inlet are colder than the ambient temperature. In fact, since the water that circulates in the system is not heated by the heat exchanger at the manifold, the inlet temperature could register low values. The inlet temperature grows more or less constantly. Instead it is possible to notice that the outlet temperature grows more quickly than the inlet temperature. That is obvious and it depends by the direct heat transferred by the heat exchanger to the out flow water. This temperature is higher than the inlet temperature. When the system has reached its regime rate, the difference of temperature, between Tin and Tout, remains quite constant. In this case, in order to reach a regime Tout curve, the collector heats the water rapidly. The system took up to 21 minutes to induce a constant difference of temperature between Tin and Tout. We can therefore say that after 21 minutes the warm up phase of the liquid, could be considered concluded. Moreover the slope of the two temperatures’ curve (T in and T out) depends by the initial temperature that in this case was really low (around 12˚C).
In the Graph 2 it is possible to identify the maximum efficiency registered by the system, which is around 44%. This graph is extremely important because in it we can see that the warm up phase is quantified in 20 minutes. The instantaneous efficiency plots a series of values that are growing quickly, and after 20 minutes the curve associated with these points has a more constant trend. We can therefore conclude that in order to identify the most precise line that identifies the characteristic of the collector we should exclude those values associated with the first warm up phase (from 0 to 21 minutes).
The most important graph is the third one. The line plotted in Graph 3 shows the efficiency curves for our collector. The vertical axis shows efficiency. The horizontal axis shows the difference between the average collector temperature and the ambient air temperature. It is clear that the efficiency is not characterized by a constant value. In fact it depends mainly by the temperature difference between the collector and the outside air temperature. As expected, the collector demonstrates a better efficiency when there is not much difference between the plate temperature Tin and T ambient. The greater the temperature differences between the collector inlet water (Tin) and the ambient air (Tambient), the lower the efficiency, due to heat losses. This is because collector losses increase when the ambient temperature is colder.
The point of intercept between the efficiency curve and the x-axis is extremely important. Considering the case when the ratio of the difference of temperature and the solar radiation, multiplied by the “heat transfer coefficient” (0.001 W/(m2K)), it is equal to the optical efficiency (0.402), than the equation of the curve efficiency (y = -0.001X + 0.402) lead to zero values. In this case the system has no efficiency, which means there is a direct conversion of solar heat into heat losses from the water tank. The fluid in fact is stable and does not move anymore. In this case we experience the “stagnation” of the system. The temperature at which stagnation occurs is called the stagnation temperature and it is a critical design parameter for a solar water heater. The intersection with the y-axis occurs when the equation of DT/I is zero. In this case the efficiency is equal to the optical efficiency.
Errors of the experiment:
Due to some approximations in the experiment, it is possible that some error may occur. Main key points are:
In reality, the ambient temperature should be calculated with a sensor located outside the building. In our case we used a thermometer positioned inside the laboratory. Moreover the sensor was extremely close to the light source, being heated by tungsten lights.
Insulation in the manifold and especially in the water tank is not sufficient to prevent accidental heat losses during the experiment. It means that heat losses are reducing dramatically the efficiency of the collector.
Tungsten lights are not powerful enough to provide irradiance up to 1000 W/m. The value we used in our calculation should be therefore reduced in order to improve the reliability of the experiment. It should be necessary to calculate the precise output of the light source. Furthermore isolation levels are never constant (like the source it has been provided to the collector in the experiment). It may fluctuate with intermittent cloud cover. In order to calculate more accurately the energy output per day/month/year a more complete set of environmental data must be considered and many (hourly) performance calculations have to be taken throughout the day.
Considering all the above limitations, the maximum efficiency recorded during the experiment (up to 44%) should be considered (in theory) higher. A typical efficiency for these kind of solar collector is between 70% and 80%.
The following picture (source http://www.solarserver.de) shows the efficiency of typical solar collectors. Among them the one with the highest efficiency is the evacuated tube solar collector.
Graph of efficiency and temperature ranges of various types of collectors (radiation: 1000 W/m²)
| Length (nominal) | 1500mm /1800mm |
| Outer tube diameter | 58mm |
| Inner tube diameter | 47mm |
| Glass thickness | 1.6mm |
| Thermal expansion | 3.3×10-6 oC |
| Material | Borosilicate Glass 3.3 |
| Absorptive Coating | Graded Al-N/Al |
| Absorptance | >92% (AM1.5) |
| Emittance | <8% (80oC) |
| Vacuum | P<5×10-3 Pa |
| Stagnation Temperature | >200oC |
| Heat Loss | <0.8W/ ( m2oC ) |
| Maximum Strength | 0.8MPa |
Handouts and lecture notes from solar thermallectures (Renewable Energy 1 course. Prof. MrGillot)
“Types of Solar Collectors” by Bob Ramlow with Benjamin Nusz
“Solar Energy Utilization” by H. Yuncu, E. Paykoc and Y. Yener
Author : Enrico Crobu
Date of the laboratory: 24/11/2008
Pictures:
Picture1; source: ‘Types of Solar Collectors’ by Bob Ramlow with Benjamin Nusz
Picture2; source: http://www.sunmaxxsolar.com
Picture 15; source: http://www.earthwindfire.co.uk/Panels/panels.html
Picture 16; source: http://www.five-shades-of-green-energy.com/solar_collectors.html
N.B. Where not indicated, pictures and diagrams are property of the Author
Links:
Appendixes
TABLE 1: Records of temperatures captured during the experiment. Source information’s to estimate the efficiency of the system being analysed.
TABEL 2: general information used for calculations.
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