|A properly designed enclosure can improve installation efficiency and stability
Credit: Emerson Process Management
In the fifth of a series of articles on optimising cogeneration plant, Dr Jacob Klimstra explains how the setup of an installation’s enclosure can affect its efficiency.
Most cogeneration installations are housed in an enclosure, which can be a container or a special boiler house. An enclosure is generally needed to avoid noise radiating to the environment and to shield sensitive equipment from the weather. An enclosure can also prevent unqualified people from tampering with the installation.
The efficiency of a cogeneration plant is also affected by its enclosure, depending on the ambient temperature and the way the ventilation system works. If no enclosure is present, the intake air of the reciprocating engine or gas turbine equals the ambient temperature. In that case, for an exhaust temperature controlled at a fixed value, a lower ambient temperature results in higher losses in sensible heat with the intake air.
In addition, the temperature difference between the engine and its surroundings will then vary with the ambient temperature, affecting the convection loss. The temperature of the cylinder block is normally thermostatically controlled.
Here we will use a series of examples to illustrate the effect of enclosures on the efficiency of a cogeneration installation.
Cogeneration efficiency with no enclosure
The fuel efficiency of a cogeneration installation is 100% where the exhaust temperature equals the intake temperature and the insulation ensures that no heat escapes from the machine to the environment. Complete combustion should also be present, and the heat coming from the electricity generator should not be lost to the environment. Such stringent boundary conditions are difficult to implement in a practical installation.
A reference temperature of 25°C is a suitable value for the sensible heat of the intake air and exhaust gas, since it is, in practice, the lowest possible temperature used in heating systems. At 25°C, heat can be used for soil heating systems in horticulture and for under-floor heating systems in buildings. When determining the heat balance of a cogeneration installation, we need to calculate how much the fuel gas and intake air must be heated or cooled to reach the reference temperature. The same applies for the exhaust gas.
Here, as we are focused on the enclosure’s effect on the energy balance, heat loss resulting from an exhaust temperature higher than the reference temperature of 25°C will not be discussed. (This was the subject of an article in the November–December 2014 issue of COSPP, now available on-line at www.cospp.com.)
The sensible heat per unit of calorific value Especific of the fuel gas equals (eq. 1):
in which cp = specific heat in kJ/kg; Tref = reference temperature in °C; Hi = lower calorific value in MJ/kg.
For methane, cp is about 2.2 kJ/kg and Hi is 50 MJ/kg for a reference temperature of 25°C. The cp of nitrogen (N2) is about 1.04 kJ/kg and that of carbon dioxide (CO2) about 0.84 kJ/kg. Some natural gases contain considerable amounts of nitrogen. Biogas can contain much CO2.
It is generally presumed that natural gas from underground pipelines has a temperature of 15°C. The specific sensible heat needed to raise the gas temperature from 15°C to 25°C is 2.2 · (25–15)/50 = 0.44 kJ/MJ, equalling 0.044% of the fuel energy, meaning that it is close to negligible. A cogeneration installation’s enclosure has no effect on the gas temperature, and therefore we will ignore the small effect of the gas temperature on efficiency.
The composition of natural gas depends on its source, but methane is always by far the main constituent. The properties of natural gas, such as calorific value and stoichiometric air requirement, depend on its composition. For simplification purposes, we will here use pure methane as our fuel gas example. The influence of the actual gas composition on an enclosure’s effect on total energy efficiency is very small.
Stoichiometric combustion of methane requires 17.36 kg of air per kg of gas. Most prime movers in cogeneration installations use a fuel-lean mixture for better performance and lower NOx emissions. For an air-to-fuel ratio λ of 2 with respect to a stoichiometric mixture, one needs 34.72 kg of air per kg of methane.
The specific sensible heat to be added to the intake air is (eq. 2):
The cp of air at ambient conditions is about 1 kJ/kg.
We now know how to determine the sensible heat required to bring the intake air and fuel gas to the reference temperature of 25°C. This amount of heat, as a percentage of the fuel energy required to raise the intake air temperature, is shown in Figure 1.
Figure 1 reveals that a cogeneration system which draws its combustion air directly from outside can need up to 6% of the fuel energy to bring the intake temperature from -30°C to the reference temperature of 25°C. Most gas turbines operate at an air-to-fuel ratio λ of 3 and higher. Modern reciprocating gas engines operate at a λ between 1.8 and 2.1.
|Figure 1. A low intake temperature requires much energy to heat up the intake air to 25°C|
The cylinder block of a reciprocating engine is often controlled at a temperature close to 85°C. This warrants a proper temperature distribution of the engine’s inner parts, while the coolant provides a suitable temperature for heating systems. For a modern turbocharged engine with a brake mean effective pressure at full load of about 20 bar, the heat loss from the engine block to its surroundings is about 1.5% to 2% of the fuel energy where the room temperature is 30°C. The actual value depends on the size and construction of the engine. Since, in a normal situation, the heat transfer from the engine block to its surroundings is via convection, the heat loss to the surroundings is directly proportional to the temperature difference. This heat loss can be written as (eq. 3):
Figure 2 shows the dependence of the convection loss on the temperature of the surroundings, in a case where no forced air flow is present around the machine. Some packagers design the ventilation system in such a way that blowers create high air flows against the engine block. This can substantially increase the engine block’s heat loss.
|Figure 2. The convection loss of a reciprocating engine is directly proportional to the temperature difference between the engine block and its surroundings|
Figures 1 and 2 show that a cogeneration installation exposed to ambient air can easily lose 6% of its fuel energy if the ambient temperature is very low. Unfortunately, heat demand generally increases with a lower ambient temperature.
An enclosure with a controlled internal temperature
As mentioned earlier, cogeneration installations are normally inside an enclosure. This includes the electricity generator. At full load, the generator loss is some 1.5% of the fuel energy. Generators have to be cooled in order to avoid overheating of their windings. Sometimes ambient air will be used to cool the generator, but in many cases the air inside the installation’s enclosure is cold enough to cool the generator.
Intake air from outside the enclosure
Figure 3 is a schematic representation of a cogeneration system in an enclosure. The temperature inside the enclosure is controlled by a variable-speed ventilator. If ventilation was not present, the enclosure would reach a temperature of over 85°C, which means that the air inside the enclosure would have the same temperature as the engine block. Therefore, convection from the engine block to its surroundings would stop.
|Figure 3. A cogeneration installation in an enclosure with forced ventilation to keep the inner temperature at 30°C.|
However, there is still heat input from the generator and from radiation of the turbocharger, and ultimately the engine block will even start to receive heat from its surroundings. Only a few examples exist where no ventilation takes place in the enclosure. This has, e.g., been the case with the FIAT TOTEM, a 15 kW cogeneration unit with a water-cooled generator. All larger installations use ventilation within the enclosure.
In the example of Figure 3, the inner temperature of the enclosure is controlled at 30°C. This means that the temperature difference between the engine block and its surroundings is constant, resulting in a convection loss at full load of some 1.5% of the lower calorific value of the fuel (equation 3). The generator loss of about 1.5% of the fuel energy is also added to the ventilation air, so that in total 3% of the fuel energy leaves the enclosure with the ventilation air.
|Cogeneration plant in Lubmin, Germany
The air for the engine process is drawn from outside, which means that the energy required to heat the intake air to the reference temperature of 25°C is dependent on the ambient temperature. Figure 4 gives the resulting fraction of the fuel energy that is lost due to intake air heating and enclosure ventilation.
Actually, the energy required to heat the intake air is exactly the same as in Figure 1. No line for λ = 3 has been given in Figure 4, since the data on the convection loss apply for reciprocating engines only and these engines do not operate at such high lambda values as the turbines. In this solution, the convection loss will never exceed 1.5% at full load. This is in contrast with a no-enclosure situation, where the convection loss can reach 3% of the fuel energy in the case of very low ambient temperatures.
|Figure 4. The heat loss for the intake air and ventilation air of a reciprocating engine-driven cogeneration installation running at full load in a temperature-controlled enclosure, expressed as a percentage of the lower heating value of the fuel|
Again, the heat from a cogeneration installation is generally most needed when the ambient temperature is very low. Therefore, this solution with intake air taken from outside the enclosure is not ideal.
Intake air from inside the enclosure
A better solution is to draw the required intake air for the engine from the enclosure itself. Figure 5 illustrates this concept. The air inflow into the enclosure is now the sum of the intake air for the engine and the ventilation air. If the average temperature inside the enclosure is below 30°C, the ventilator will stop, and the heat from the generator and engine block will only heat the inflow of air which exactly equals the intake air of the engine.
|Figure 5: A cogeneration installation that draws its combustion air from inside its enclosure|
The question now is which average temperature will be present in the enclosure, depending on the ambient temperature. If the air flow into the enclosure equals the intake air flow, the energy needed to heat the inflow of air to the enclosure temperature equals, for a λ of 2:
The heat provided from the engine block (see equation 3) and the generator (= 1.5%) equals:
Therefore, as long as the temperature inside the enclosure is lower than 30°C:
The result of this relationship is shown in Figure 6.
Figure 6 immediately shows the huge advantage of drawing the intake air from inside the enclosure, instead of directly from the ambient air. In the previous case, with intake air drawn from outside the enclosure, almost 4% of the fuel energy is needed to heat the intake air from -30°C to + 25°C, while 3% of the fuel energy resulting from engine block and generator losses had to be ventilated away. In the case of drawing the intake air from inside the enclosure, for the same low temperature of -30°C, only about 0.5% of the fuel energy is needed to raise the intake air to the reference temperature of 25°C. Therefore, for ambient temperatures below -12°C, the energy efficiency of a cogeneration plant is about three percentage points better when the air for the engine is drawn from inside the enclosure.
|Figure 6. The enclosure temperature as a function of the ambient temperature where the engine intake air is drawn from the enclosure|
A positive side effect of taking the intake air from inside the enclosure is that its temperature varies only slightly with the ambient temperature. This makes the task of the lambda control system much easier, since the temperature of the intake air considerably affects the air-to-fuel ratio prepared by carburettors.
Finally, figure 7 shows how much heat has to be ventilated away when the intake air for the engine is drawn from inside the enclosure at ambient temperatures from -12.5°C. If the ambient temperature exceeds 30°C, the ventilation system is no longer able to keep the temperature in the enclosure at 30°C. For ambient temperatures exceeding 30°C, the convection loss from the engine block will begin to decrease.
|Figure 7. The fraction of fuel energy ventilated away as heat from a cogeneration installation, where the intake air is taken from inside the enclosure and the set point for the enclosure temperature is 30°C|
The output of turbocharged reciprocating engines does not depend on the intake air temperature as long as this temperature remains below 30°C. This is in contrast with gas turbines, where the power capacity is proportional to the intake air temperature.
In practice, the temperature inside an enclosure is not always uniform. In some designs, ventilators push ambient air into the enclosure instead of drawing the air out. In such situations, the engine’s intake air can have a temperature close to the ambient temperature. The convection heat and the generator loss are then not used to heat the intake air, resulting in less-than-optimum fuel efficiency. High convection losses also occur when cold ambient air is blown against the engine block. Sometimes incoming air jets from the ventilators are the cause of a heavily fluctuating intake-air temperature, which creates unsteady engine operation. It is recommended here to keep the atmosphere inside an enclosure as quiet as possible, with ventilators drawing the air from the enclosure. A slight underpressure inside the enclosure also prevents foul gases escaping when the doors are opened during running.
A properly designed enclosure can clearly improve a cogeneration installation’s energy efficiency and stability, especially on cold days.
Dr Jacob Klimstra is Managing Editor of COSPP