Abstract. Combined power generation or cogeneration is highly effective technology that produces heat and electricity in one device more efficient than separate production.Overall efectivity is growing by use of combined technologies of energy extraction - taking heat from flue gases and coolants of machines. Another problem is the dependence of such devices on fossil fuels as fuel for combustion turbines are the most common natural gas, kerosene and fuel for heating plants is coal. It is therefore necessary to seek for compensation today, which confirms the assumption in the future . At first glance, the obvious efforts to restrict the use of largely oil and change the type of energy used in transport . Another significant change is the increase in renewable energy - energy that is produced from renewable sources . Between machines gaining energy by unconventional way are belonging mainly steam engine , Stirling engine and Ericsson engine. In these machines , the energy is obtained by external combustion and engine performs work in a medium that receives and transmits energy from combustion or flue gases undirectly . The article deals with the principle of hot-air engines, their use in combined heat and electricity production from biomass and with heat exchangers as primary energy transforming element.