**1. Introduction**

More efficient energy consumption, and thus reduced consumption of nonrenewable energy sources, can significantly reduce energy costs, mitigate climate change, improve the quality of life and reduce the EU's dependence on imported oil and gas. To achieve these goals, energy efficiency must be improved throughout the energy chain, from production to final consumption. EU action therefore focuses on sectors where savings can be greatest, such as energy consumption for heating and cooling buildings. In 2007, the EU set three main targets: a 20% reduction in greenhouse gas emissions (compared to 1990 levels), 20% of energy consumption from renewable sources in the EU and a 20% improvement in energy efficiency.

The 20% energy efficiency target was adopted with the adoption of Energy Efficiency Directive 2012/27/EU in 2012 [1].

The development of energy consumption since 2014 shows that the EU's energy consumption targets for 2020 have not been met. The crisis of COVID has severely affected the economy, reducing energy consumption in 2020. However, unless the European economy becomes more energy efficient, the subsequent economic recovery will lead to a resurgence of energy consumption. EU Member States have set up a working group to discuss with stakeholders the reasons for the increase in energy consumption in 2014 and 2017 and possible measures to address the problem.

The new edition of the Energy Efficiency Directive (EU) 2018/2002 entered into force in December 2018. The directive contains several new elements and some updates to previous directives. The EU's main goal is to achieve 32.5% energy efficiency by 2030 (compared to forecasts of expected energy consumption in 2030), with a clause on a possible upgrade by 2023. In accordance with the Energy Union and Climate Action Regulation (EU) 2018/1999, each Member State must draw up an integrated national energy and climate change plan (NECP) for the period 2021–2030, covering a 10-year period from 2021 to 2030 and describing how it intends to contribute to the objectives for 2030 in terms of energy efficiency, use of renewable energy sources and greenhouse gas emissions [1].

Moreover, increasing the efficiency of new or existing heat generation plants is one of the priorities in line with the EU's commitments to reduce GHG (greenhouse gases) emissions and achieve several environmental goals [1].

With the development of techniques and the growing demand for energy, more emphasis is now being placed on the use of renewable energy sources, in line with EU directives and the adoption of new legislation. The focus is on the rational use of energy and energy self-sufficiency of commercial and public buildings with all types of energy (electricity, heating and cooling).

There are only a few studies dealing with the coupling of heat pumps and CHP (combined heat and power) engines. An experimental study was conducted to increase the heating capacity of an electric heat pump using heat recovered from the generator of a gas engine [2]. Mancarella presented an approach for energy and CO2 emission modeling of CHP systems coupled with electric heat pumps [3], Blarke and Dotzauer [4] developed a novel CHP concept with a compression heat pump and cold storage using exhaust heat. Similar concepts were presented in [5], where Blarke compared an electric boiler and heat pumps with respect to decentralized CHP in West Denmark. Capunder et al. [6] presented an optimization model to evaluate the techno-economic and environmental characteristics of different multi-generation options.

The objectives for the use of surplus low temperature energy sources in CHP gas engines are:


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

The purpose of using redundant low- temperature sources of CHP gas engines is:

