**5. Economics of excess heat recovery of CHP gas engines**

The economics of the exploitation of low-temperature heat sources from a selected CHP gas engine with the four HTHP, was calculated in MS Excel. For the HTHP drive, instead of the electricity produced by the CHP device, we can use electricity from the electricity grid.

The price of electricity, when we take it from the grid, is made up of production costs, transport costs, distribution costs, nominal power, various taxes, etc. and averages 95.2 €/MWh. The price of electricity produced in combined heat and power generation and sold on the electricity market averages 43.5 € /MWh. For the


#### **Table 4.**

*Basic operating data of the high-temperature heat pump (HTHP), energy prices and other economic data.*

economic calculation of the surplus heat recovery of CHP gas engines with HTHP, electricity prices vary from country to country.

Two calculations have been made:


Basic technical and economic data are given in **Table 4**. The average price of heat was defined as the production price of heat produced from natural gas with CHP device and gas boilers district heating system.

The economic calculation of the exploitation of low-temperature heat sources from a CHP plant with the HTHP using electricity from a CHP plant (43.5€/MWh) is shown as a diagram in **Figure 7**.

**Figure 7.** *Cumulated discounted cash flow at the price of the electricity at 43.5 €/MWh.*

#### **Figure 8.**

*Cumulated discounted cash flow at the price of the electricity at 95.2 €/MWh.*

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

The diagram in **Figure 7** shows the internal rate of return IRR = 39.4% and payback of the investment exploiting excess heat from CHP device with four seriesconnected high-temperature heat pumps is approximately 2.5 years.

The economic calculation of the exploitation of low temperature heat sources from the CHP device with four series-connected high-temperature heat pumps with the electricity taken from the grid (95.2 €/MWh) is shown as a diagram in **Figure 8**.

The diagram in **Figure 8** shows the internal rate of return IRR = 30.5% and payback of the investment exploiting excess heat from CHP device with four seriesconnected high-temperature heat pumps is approximately 3.3 years.

## **6. Conclusions**

An innovative technical solution for increasing the efficiency of the primary fuel and thus reducing CO2 emissions by retrofitting existing or new CHP gas engines with high-temperature heat pumps is presented. High-temperature heat pumps use the excess heat of the exhaust gasses and the heat of the cooling system of the CHP gas engine and heat the return water of the district heating system.

The described principle of using the excess heat of CHP gas engines can also be used for the use of low-temperature heat sources, including hot water boilers and other types of CHP equipment.

In order to use the excess heat of the exhaust gasses and the cooling system of the CHP gas engine, it is necessary to install a heat exchanger in the exhaust system, where they are further cooled by cooling and condensation of the water contained in the exhaust gasses. The heat generated in this way is too low to be used directly for high-temperature heating, but it can also be used to heat the return water of the district heating system by using high-temperature heat pumps.

To illustrate how the innovative technology of using low-temperature sources of CHP gas engines works, a computer simulation of four series-connected high-temperature heat pumps was carried out using the Aspen plus software package. With a system of series-connected high-temperature heat pumps, the overall efficiency of natural gas can be increased by 17.6% (from 86.8% to 104.4% in terms of LHV of natural gas) by using low-temperature heat sources of CHP gas engine. An increase of 17.6% in the primary energy efficiency of the natural gas CHP gas engine is achieved when the exhaust gasses from the gas engine are cooled to a temperature of 25°C.

The energetic efficiency of the primary fuel of the CHP gas engine can be significantly increased to approx. 117% in relation to the LHV of natural gas by additional cooling of the exhaust gasses (up to approx. 5°C) and utilization of the heat emitted from the surface of the CHP gas engine to the environment. The temperature to which the exhaust gasses would be cooled depends on the economic efficiency of the operation of high-temperature heat pumps, because when the temperature of a low-temperature source drops, the average COP of heat pumps drops rapidly.

The presented technical solution for increasing the primary fuel efficiency of the CHP gas engine by using the excess heat of the exhaust gasses and the cooling system of the gas engine is also very economical with a very short return on investment.

During the computer simulation of the presented technical solutions for increasing the efficiency of the primary fuel of the CHP gas engine using high temperature heat pumps, the operating parameters of the manufacturer of the CHP gas engine and the high temperature heat pump were taken into account.

*Alternative Energies and Efficiency Evaluation*
