**4. Process simulation with AspenPlus software package**

In order to be able to use the energy of the exhaust gasses at a temperature of 120°C, a retrofit in the form of an additional heat exchanger (HE2) in the exhaust gas system is required, which would cool the exhaust gasses to 25°C. In this way, 1,100 kW of heat could be recovered. The second low temperature heat source is the cooling system 2nd stage intercooler (HE3) where 197 kW of heat could be recovered. The temperature of both low-temperature heat sources is too low to be used directly for high-temperature heating, but can still be used by using the HTHP, which raises the temperature level to a level suitable for high-temperature heating. A high temperature heat pump (or several of them) is integrated into the CHP system, as shown in **Figure 1**, to use the low temperature heat exchangers HE2 in HE3. The heat flow recovered from the high temperature heat pump is used to reheat the water return to a temperature of 70°C. In case the return water temperature is too high, the heated water is sent directly to the supply line through valve V1 - **Figure 1**. The total low-temperature heat flow obtained by the CHP unit with a nominal capacity of 3.3 MW with heat exchangers HE2 and HE3 is approximately 1,297 kW. To utilize the 1,297 kW from the low-temperature heat source by heating the return water from the high-temperature heating system from 60–70°C, which is the maximum water temperature allowed to enter the CHP unit, would require four high-temperature heat pumps with a rated capacity of 500 kW. The low-temperature energy source of HE2 and HE3 is utilized with the circulation of a glycol-water mixture, to which four high-temperature heat pumps are connected in sequence.

To utilize the excess low-temperature heat from the CHP gas engine, whose operating data are given in **Table 3**, a computer simulation of four series-connected high-temperature heat pumps with a rated capacity of 500 kW was performed using the Aspen Plus software package. The operating data of a 500 kW high temperature heat pump at a compressor speed of 1450 rpm are given in **Table 3**. Heat generation with a 500 kW high temperature heat pump can be modified with a frequency controlled electric motor drive of a reciprocating compressor with a maximum permissible speed of 1600 rpm.

**Figure 5** schematically shows the serial connection of four high-temperature heat pumps with some results of computer simulation. Four series-connected hightemperature heat pumps consist of four compressors (COMP1, COMP2, COMP3, COMP4), four refrigerant evaporators (EVAP1, EVAP2, EVAP3, EVAP4), four condensers (COND1, COND2, COND3, EXP3) and four expansion valves (VALVE1, VALVE2, VALVE3, VALVE4). The low-temperature heat source of four seriesconnected high-temperature heat pumps is water or a mixture of water and glycol heated in a heat exchanger (HE2 and HE3), shown in **Figure 1**.

Water or a mixture of water and glycol, first heated slightly to 50°C in a heat exchanger HE2 and HE3 and fed through line P9 and P14 shown in **Figure 1** and **Figure 5** successively through all four evaporators (EVAP1, EVAP2, EVAP3, EVAP4). Chilled water or a mixture of water and glycol at about 23°C leaving the EVAP 4 evaporator is returned to the heat exchanger HE2 and HE3 via line P8 and P15. In each of the four evaporators of high-temperature heat pumps, the evaporation pressure of the refrigerant (ammonia) is different because it depends on the temperature and the available heat flow obtained in each individual evaporator. Refrigerant vapors leaving the evaporators are compressed by compressors

**Figure 5.**

*The scene of four series-connected high-temperature heat pumps with the results of computer simulation using the Aspen plus software package.*

(COMP1, COMP2, COMP3, COMP4) to the pressure required to condense the refrigerant in series-connected condensers (COND1, COND2, COND3, COND4) when heating the district heating return water.

The return water of the district heating system is fed to the first condenser via pipe P4, as shown in **Figures 1** and **5**, and leads sequentially to each individual condenser of the high-temperature heat pump. In each condenser, the heating water heats up a little until the desired temperature is reached in the last condenser. The water of the district heating system heated in this way is then fed to the CHP gas engine via pipe P5, in which it is heated to the final temperature of the district heating system.

To compare the operating characteristics of the system of four series-connected high-temperature heat pumps for the exploitation of low-temperature CHP gas engine heat sources, a computer simulation of one high-temperature heat pump with the same operating characteristics as four parallel-connected 500 kW hightemperature heat pumps is made.

**Figure 6** shows a diagram of one high-temperature heat pump consisting of a single compressor (COMP), a refrigerant evaporator (EVAP), a condenser (COND) and an expansion valve (VALVE). The low temperature heat source of a high

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

**Figure 6.**

*Schematic diagram of a high-temperature heat pump with the results of a computer simulation using the Aspen plus software package.*

temperature heat pump is water, or a mixture of water and glycol heated in the heat exchanger HE2 and HE3 shown in **Figure 1**. The water or mixture of water and glycol is heated to about 50°C and through line P9 and P14 shown in **Figures 1** and **6**, is fed to a high temperature heat pump evaporator (EVAP), where it is cooled to about 23°C and returned to the heat exchanger HE2 and HE3.

Refrigerant vapors leaving the evaporator (EVAP) are compressed in the compressor (COMP) to the pressure required to condense the refrigerant in the condenser (COND) when heating the return water of the district heating system. In the condenser, the return water of the district heating system is heated to the desired temperature and then returned to the CHP gas engine via pipe P5, where it is heated to the final temperature.

A summary of the results of the computer simulation of the four high-temperature heat pumps connected in series in **Figure 5** is shown in **Table 1**. The computer simulation results presented in **Table 1** show that the average COP of all four high temperature heat pumps is 5.81. To drive the frequency-controlled electric motor drives of the compressors in all four high-temperature heat pumps, 269 kW of electricity would be required. By lowering the temperature of a low-temperature heat source, the COP and the output of the series-connected high-temperature heat pumps also decrease. The COP of high-temperature heat pumps also decreases as the district heating return heating water increases. The calculation was made by heating the heating return water from a temperature of 60°C to a temperature of 70°C in series-connected condensers of four high-temperature heat pumps.

A summary of the results of the computer simulation of a high temperature heat pump in **Figure 6** is shown in **Table 2**. The results of the computer simulation in


#### **Table 1.**

*Summary of the results of the computer simulation of the four series connection of high-temperature heat pumps from Figure 5.*


**Table 2.**

*Summary of the results of the computer simulation of the single high-temperature heat pump from Figure 6.*

**Table 2** show that the average COP value of a high temperature heat pump is 4.21, which is much less than four serial connected high temperature heat pumps. The reason for this is that it is necessary to overcome the greater temperature difference between the outlet temperature of the low temperature heat source and the desired temperature of the heating water return of the heating system with a high temperature heat pump. To drive the frequency-controlled electric motor drive of a


#### **Table 3.**

*Technical data for CHP device [6] rated power 3.3 MW and technical data for CHP device rated power 3.3 MW with series-installed high-temperature heat pumps [7].*

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

compressor of a high-temperature heat pump, a higher electrical power (403 kW) is therefore required, since a higher-pressure ratio between the evaporator pressure in the evaporator and the condensing pressure in the condenser must be created.

The energy consumption efficiency of natural gas and the utilization of excess low-temperature heat of a CHP gas engine with four series-connected high-temperature heat pumps are given in **Table 3**.

Electricity is needed to drive the frequency-controlled electric motors of the high-pressure compressors of high-temperature heat pumps. The electricity can be drawn from the grid or electricity from a gas engine with combined heat and power generation can be used. In **Table 3**, we have used 269 kW of electricity generated by a CHP unit to drive four high-temperature heat pumps connected in series, so that the electrical efficiency of the CHP fell from 45.6% to 41.9%. The total heat produced by the CHP and the four CHP units to use the excess low temperature heat from the CHP gas engine increased from 41.3% to 62.5%. The overall energy efficiency of the primary fuel of the CHP was increased by 17.6%, from 86.8% to 104.4%.
