**2. Operating the conventional CHP device**

The CHP plant enables the simultaneous production of heat and electricity within one unit. The unit converts the chemical energy of the fuel by means of a steam turbine, gas turbine or internal combustion engine into mechanical energy, which is driven by a generator via a shaft, which converts the invested mechanical energy into electrical energy. A byproduct of this process is also the useful heat for high-temperature heating and the low-temperature waste heat, which is released into the environment via the cooling system and the exhaust system [6, 7].

The need for operation of the CHP plant is influenced by the season, the outdoor temperature and the specific needs of the end users. The mode of operation changes accordingly and adapts to the current demand for useful heat and electricity. In relation to the amount of usable heat and electricity, a proportional share of excess low-temperature heat is also produced.

## **3. Exploitation of excess low-temperature heat sources from CHP gas engines**

When operating CHP gas engines, many low-temperature heat sources are generated, which are released into the environment via the cooling and exhaust system of the gas engine. These low- temperature heat sources are too low to be used directly to heat the return water of the district heating system. To be able to use this low-temperature heat source of the CHP gas engine, it is necessary to raise the temperature to the required temperature level using a high-temperature heat pump or several high-temperature heat pumps connected in series.

The principle of using surplus low-temperature heat sources of the CHP gas engine with built-in high-temperature heat pump is shown in **Figure 1**, where they are shown and marked: drive unit for combined heat and power (CHP), gas engine (ICE) with internal combustion, generator (G) for electricity generation, heat exchangers (HE1 - HE3), high-temperature heat pump (HP), piping system (P1 - P19) for heat distribution, heat consumer (HC), valves (V1 - V3), dampers

**Figure 1.** *The principle of exploiting the low-temperature sources of the CHP plant using the high-temperature heat pump [7].*

(H1 - H4), fan (F1), pumps (PU1 - PU4), backup cooling system (CT1), external environment (O) and temperature sensors (T1 - T13) [7].

The drive unit of the combined heat and power plant (CHP) is directly connected to the high temperature district heating system of the end user (HC) with integrated cooling system or heat exchanger with piping (P1 and P3). The primary medium or heat transfer medium in a high-temperature district heating system is water, which transfers the thermal energy of the combustion engine cooling system to the end consumer (HC) of the high-temperature district heating system.

The operation of a combustion gas engine (ICE) produces hot exhaust gasses which are discharged through the pipe (P16) into two heat exchangers (HE1 and

#### *Exploitation of Excess Low-Temperature Heat Sources from Cogeneration Gas Engines DOI: http://dx.doi.org/10.5772/intechopen.98369*

HE2) connected in series. The first of the heat exchangers (HE1) on the exhaust system is designed to use the high-temperature heat of the exhaust gasses, which it transfers directly to the primary medium of the high-temperature heating system. After the high-temperature heat is dissipated in the heat exchanger (HE1), the exhaust gasses continue their path through the pipe (P18) to the heat exchanger (HE2). Due to the low temperature of the exhaust gasses in the pipe (P18) and the heating of the primary medium in the pipe (P8) (HE2), the heat of the primary medium is not suitable for further direct use in a high temperature heating system. To use this low-temperature source, a high-temperature heat pump (HP) is therefore used, into which the condenser directs the return flow of the district heating system. The heat exchanger (HE2) heats the secondary medium (water or a mixture of water and glycol) by further cooling the exhaust gasses and condensing the water contained in the exhaust gasses. This medium circulates in a closed circuit between the heat exchanger (HE2) and the evaporator of the high-temperature heat pump (HP). In the evaporator of the high-temperature heat pump (HP) the secondary medium evaporates the refrigerant of the heat pump (HP).

In a similar way, the low-temperature heat source intercooler 2nd stage and gas engine lubricating oil (ICE) is used via a pipe connection (P10 and P11) to the heat exchanger (HE3).

Due to the use of this low-temperature heat source, the evaporator of the hightemperature heat pump (HP) is connected to a secondary heat exchanger (HE3), which contributes part of the low-temperature heat to the evaporation of the working medium in the evaporator of the high-temperature heat pump (HP). One or more heat exchangers can be integrated in the series or parallel connection of CHP gas engines and high temperature heat pumps to use the excess low temperature heat sources which are now released into the environment.

To illustrate the process of using low-temperature heat sources from CHP gas engines, data on the operation of the commercially available CHP plant were obtained [8]. In **Table 3** nominal power of the 3.3 MW CHP plant given. The estimated operating time of the CHP plant depends on the heat consumer's demand. In the presented example are about 4000 h/year. The operation mode of the district heating pipe network or HC heat consumers is 90/60°C in winter and 90/55°C in summer.

## **3.1 Excess low-temperature heat from the CHP gas engine**

The excess low temperature heat of the CHP gas engine is released unused to the environment in several ways. The most common are:


#### **Figure 2.**

*The obtained low-temperature heat flow with an additional cooling of the flue gasses from the CHP gas engine [9].*

In order to use the low temperature heat of the exhaust gas with a temperature of 120°C, it is necessary to install the condenser heat exchanger HE2 (**Figure 1**) in the exhaust system of the gas engine, where the exhaust gases should be cooled down to a temperature of 25°C. For this purpose, a computer simulation of the cooling of the exhaust gases was carried out using the Aspen plus software, the results of which are shown in **Figure 2** [9].

The heat flux obtained with additional cooling of the flue gasses from the CHP unit is presented on **Figure 2**. the specifications of which are shown in **Table 3**, where the flue gasses are first cooled from a temperature of 120°C to a temperature of 46°C, at which the water from the flue gasses starts to condense in the condenser heat exchanger HE2 (**Figure 1**). **Figure 2** shows the mass flow of condensed water from the flue gas as a function of the temperatures of the flue gasses.

The diagrams in **Figure 2** show that the CHP exhaust gas are first cooled from a temperature of 120°C to a temperature of 46°C, yielding approximately 400 kW of heat. However, further cooling of the exhaust gases causes condensation of water vapor, which is present in the exhaust gases. Most of the heat, about 700 kW, is obtained by cooling the exhaust gases from 46–25°C, producing about 0.250 kg/s of condensate.

By further cooling the exhaust gases of the CHP gas engine to about 5°C, an additional 600 kW of heat could be obtained. By exploiting the heat released from the surface of the CHP gas engine and heating the air in the room where the CHP device is installed, an additional 202 kW of heat could be obtained. This means that by further cooling the exhaust gases from 25–5°C and utilizing the heat released from the external surfaces of the CHP unit, approximately 800 kW of low-temperature heat could be extracted, which could be utilized by a high-temperature heat pump and thus further increase the primary fuel efficiency of the CHP gas engine to about 117% relative to the LHV of natural gas. The temperature to which the flue gases would be cooled depends on the economics of operation of high-temperature heat pumps, because as the temperature of the low-temperature source decreases, the average COP of the heat pumps decreases rapidly. By lowering the evaporation temperature of the refrigerant in the evaporator of the high-temperature heat pump, the pressure ratio of the compressor increases, so more power is required for the electric motor drive of the compressor, which results in a lower COP.

*Exploitation of Excess Low-Temperature Heat Sources from Cogeneration Gas Engines DOI: http://dx.doi.org/10.5772/intechopen.98369*

#### **3.2 High-temperature heat pump**

High-temperature heat pumps have a high added value and contribute a lot to energy dependence reduction. They can be used in all industries where waste heat flows of different fluids are generated. They allow an economically and ecologically efficient use of low-temperature resources to improve specific energy use in processes, increase efficiency and consequently reduce CO2 emissions through the reduced consumption of fossil fuels for heat generation. The high-temperature heat pump has created the possibility of using heat from renewable or non-renewable low-temperature energy sources to meet the needs of technological processes or high-temperature heating systems.

Heat pumps have been around for many years, about as long as refrigeration units. The rapid development of heat pumps was triggered by the first oil crisis. People then began an intensive search for a replacement for fossil fuels and corresponding technological solutions. Laws related to pollution became stricter, people became aware of pollution and its effects, and energy prices increased. Heat pumps became popular due to their energy efficiency and environmental friendliness. In most of the cases, heat pumps were used for cooling purposes, while they were used for heating buildings only in case of low temperature heating up to 60°C. With the high temperature heat pump, low temperature heat sources up to 55°C can be used so that the heat potential is used to produce hot water up to 85°C [10]. This heat can be used for heating buildings or in industrial processes, and simultaneously cold water can be produced (down to 10°C), for air conditioning needs [11, 12].

The single-stage high temperature heat pump operation is based on the deprivation of heat from a low-temperature fluid (water) to get it to a higher temperature level.

For high-temperature heat pumps different low-temperature heat sources can be used:


High-temperature heat pump efficiency is determined by a heating number, which is the ratio between the heat flow generated in the condenser for heating requirements and the electricity consumed to drive the compressor. The evaporator heat flow indicates how much heat was generated from the low temperature energy source and the condenser heat flow indicates how much heat was generated for heating purposes. The determination of the power required to drive the compressor allows the determination of the power consumption for the compression of the refrigerant, which is a substance with special physical properties [13]. The use of the high temperature heat pump allows:

• Use of low temperature heat sources in an area where the infrastructure for high temperature heating already exists,


The operating characteristics of the 500-kW high-temperature heat pump as a function of the speed of a compressor, the required temperature of the output water, and the water temperature of a low-temperature source are given in **Figures 3** and **4** [10]. The results are given for different operating conditions using the working fluid R717 (NH3) and the commercial 50-bar piston compressor for the hot water temperatures from 65 to 85°C. Other refrigerants were found to be less suitable due to lower enthalpy difference between vapor and liquid

**Figure 3.** *The heat output in dependence of source inflow temperature for different temperatures of hot water.*

**Figure 4.** *The COP in dependence of source inflow temperature for different temperatures of hot water.*

phases, lower heat flux, and lower COP. The capacity of the compressor in the HTHP can be controlled in steps (970; 1,450; and 1,600 rpm) or by a stepless control of the electric motor driving the compressor.
