Good Combined Heat And Power Report Example

Introduction

The process, in which the heat produced in result of fuel combustion, simultaneously produces electricity, and heat is called cogeneration and the name of the built and configured system that utilizes the heat is Combined Heat and Power Engine (CHP). The CHP usage has so many advantages in different manners, especially nowadays, that CHP systems with up to 90% efficiency and compatible to different types of fuels, including the renewable ones are produced.
Therefore using a CHP, leads to production of the desired result but by using less fuel, and in comparison to the conventional methods it saves energy in amount of 15-40%. In result, along with the reduction in production of CO2 emission, the budget needed for fuel will be reduced too, which means this system is an environmental friendly system. In figure 1, the efficiency of a CHP system and separated heat and power production is compared and illustrated.
Figure 1: Conventional Generation vs. CHP: Overall Efficiency

Aims

The lab test was supposed to analyse the efficiency of a Combined Heat and Power Engine and the heat exchanger used for cooling of the system.

Equipment – Components

The equipment used for this test were, TOTEM which is a high-efficient cogenerator, and a software that in several important parts of the system recorded the temperature of the engine, and displayed them on a monitor. TOTEM is capable of production of 15kW of electricity and 39 kW of heat as hot water (85°C).

Its basic components are:

A 903cc fiat engine
An asynchronous electric generator
A complete heat-exchanger system
A micro-processor based system to control and monitor the system
The engine package produces the heat and electricity, and the water is heated with the help of heat exchanger and the heat produces by engine.

Procedure

The experiment has the following procedure:
After starting the engine, the electrical heating load in the water reservoir is set to 3kW
The computer software created and displayed the temperature graph, which were observed until they reached a steady state.
All temperatures shown on the monitor were recorded, along with the three flow measurements by pressing a special button. Moreover, the flow rate of the fuel was recorded by the gas flow meter. In figure 2 a schematic diagram of the water cooling system, with identified temperature zones is shown below, while figure 3 shows the interface of the software.

The same procedure for 6kW, 9kW, 12kW and 15 kW electrical loads was repeated.

Even though in the lab notes all procedure steps are clearly stated, but the used electrical heating loads were different.
Figure 2: Schematic diagram of the water cooling system
Figure 3: Software Interface

Discussion

The calculation of the efficiency was the most crucial and sensitive part of the laboratory experiment, which is the main aim of the experiment, to analyse the efficiency of the system.
The efficiency of the heat exchanger depends on the amount of energy that leaves from both sides of it. Therefore, the efficiency of the exchanger was calculated by measuring the heat that leaves the exchanger in the external loop (cools the fluid), and by measuring the heat that leave the heat exchanger in the internal loop (fluid getting cooled).
So respectively for the two “output” energies, the values were given as Qext and Qout..
The efficiency of the CHP engine (TOTEM) is defined by total output energy of the system, which includes both the heat and electric parts, and the energy supplying the system.
Thus the efficiency can be calculated by dividing of the energy produced by the system by the supplying energy of the system.

Respectively the values where given as Qout and Qin again.

The effectiveness of the heat exchanger was also calculated. The values which define the effectiveness of the exchanger may vary each time, depending on the type of fluids used and their capacity. In order to calculate the effectiveness the actual heat transfer must be divided by the Maximum possible Heat Transfer. The procedure may sound to be simple but in fact, the it is very complicated, and that’s due to the fact that lots of calculations and interpolations must be completed in order to have the energies calculations applicable. Therefore, before calculation of each energy, first, other values had to be determined, as in “Calculations” section it is clearly shown. After the calculation of the input and output energies, the efficiencies followed as well as the effectiveness of the heat exchanger. Later, the TOTEM engine’s overall efficiency was calculated that is equal to the average of those efficiencies. In addition the calculation of the electrical and heat energy output were done separately in order to perform a comparison.
As mentioned before, one of the most important parts was the procedure of the calculation of the above-mentioned energies, which acquire the needed values in the equations, as explained below:

All the required temperatures in order to do the calculations were available in the lab results.

The specific density and heat of the water were provided as constants in order to do the calculations.
For the 50% water solution of Ethylene Glycol (fluid in the external loop) the specific heat on a table and for different ranges of temperatures was given. Therefore, to calculate the values interpolation was used in order to match the temperatures of each electrical heating load.
The 50% water solution of Ethylene Glycol’s density was calculated by using the specific gravity based on water solutions, which was already provided on the table for different ranges of temperatures. Initially, through interpolation SG was calculated, and then to find the density of the solution it was multiplied by the density of the water.
The 3 required mass flow rates were recorded through the results and as current readings, which were also converted from voltage to m/s by using a equation.

Errors

In the below the possible errors that may occur in the variation of the final results are explained:
There might be a slight error in measurements of the temperature by the thermometers in various parts of the system, which means the values may be inaccurate.
The computer software refreshes each 20 seconds, and these intervals may cause a difference up to 3% in the measured values. In other words each 20 seconds there were changes in the values (temperatures, flow rate, etc.), and as there was a refresh while the values were recorder, some values could be from the new reading and some from an older one.
If each result value is rounded to 3 or 4 significant figures and then be used in another formula, then there might be a very tiny percentage of error in the results in about 0.01% to 0.1%, which cannot be considered as an error causing significant difference.
In order to record any of the values the system had to be stabilized which also can be identified from the graph. In case the system was not stable enough to receive a load of electric heat, and the values were recorded sooner than they were supposed to, depending on when the values were recorded, an error of 1 - 4% could happen in result. Just if the software had an alert system to inform the user about the stability of the system, or if it could provide the values only after an adequate stabilization of the system, then the accuracy of the recorded values could be much higher. In addition, a feature to estimate the percentage of the error of the reading of the values could be useful.
In contrast with the 50 % water solution of ethylene glycol, which was interpolated, the values of the water specific heat and density were taken as constants, which also could result in a small percentage of error.
In figure 4, the split heat and electrical energies output are compared. According to the figure, the amount of heat energy produced is much higher in comparison to the electrical energy power. In addition, by increasing the electrical heating load, we notice an increase in the heat energy output. On the other hand, after that the electrical energy output increases and reaches the electrical heating load of 11kW, it starts to decrease.
In figure 5, the input and the output energies are compared. As it is shown clearly, up to the electrical heating load of 13 kW, both output and input energies are very close. However as the as the electrical heating load reaches 13kW, the output energy starts to decrease.
Figure 4: Electrical Heating Load Vs Split
Figure 5: Electrical Heating Load Vs Energies

Conclusion

According to the analysis of the calculated results, it can be stated that the efficiency of the TOTEM is significantly high. After analysing the effectiveness and the efficiency of the split, it was shown that the CHP engine which the experiment was based on it, by inclusion of the heat exchanger demonstrated a very high efficiency almost as well as an ideal CHP engine.
Even though the results calculated could be slightly inaccurate in result of the errors mentioned before, and the actual efficiency could be less than what is calculated, undoubtedly the CHP system usage in comparison to the usage of the other conventional methods is much more efficient and effective.
In general the experiment showed that the system is highly efficient but still not all of the heat produced by the car engine is efficiently harnessed. Therefore, to increase the overall efficiency of the system, an upgrade or integration that could reduce the amount of heat loss could be highly effective. In addition, in electric part some losses were noticed too. A power generator with higher efficiency or another efficient method for transferring the electricity into the water as heat in order to reduce the power losses could increase the overall efficiency of the system.