In recent years, great attention, both in research and application fields, has been focused on the transition from centralized to decentralized (or Distributed Generation, DG) energy "production" systems. This process is currently being carried out partially. The benefits and drawbacks that DG will provide to the end-user and to the community have also been analyzed in both technical and scientific literature. All over the world researchers are strongly involved in the so-called "hydrogen economy" scenario that expects a geographically widespread system of production, storage, transportation and use of hydrogen. Furthermore, the actual industrial trend towards the miniaturization of the energy conversion equipment, due mainly to reducing manufacturing costs, results in the availability of a wide variety of small scale power, refrigeration and heat pump systems in the market. Very soon, small, micro and nano mechanical and thermal devices will be used in actual applications. In many sectors, small scale energy conversion plants (Polygeneration, Trigeneration, Combined Cooling Heating and Power) allow for the satisfaction of different energy requirements (electricity, cooling and heating) with a great potential for primary energy saving and greenhouse gas emission reduction. The "core" of these technologies is a prime mover based on different technologies (Stirling, Reciprocating Internal Combustion, Fuel Cell, Gas Turbine, ...), specially designed to operate in stationary conditions for a long time. This operation is accompanied by high efficiency output and very low pollutant emissions with regards to the reference separate "production" by large thermal power stations. At the moment, the most common technology, the gas-fired Reciprocating Internal Combustion (RIC) engine, has very good features e.g. in terms of installation space, thermal efficiency, low noise and vibration and maintenance. These engines can drive electric generators and/or electric heat pumps, absorption heat pumps and so on in different ways (mechanically, electrically and thermally), thereby allowing a wide range of operating conditions to match thermal (heating and cooling) and electric end-user requirements. The aim of this paper is to study the Energy, Economic and Environmental implications (3-E analysis) of using these complex small scale trigeneration energy conversion systems, starting with the results of an intensive theoretical and experimental research activity. In particular these systems, in comparison with conventional system, based on separate energy production, can guarantee a primary energy saving up to 28% and a reduction of equivalent CO2 emissions up to 36% when the trigeneration system is based on a small scale cogeneration system (Micro Combined Heat and Power, MCHP) coupled to a Heat Pump (HP). Satisfactory results can be achieved considering a cogeneration system which interacts with an Electric Heat Pump (EHP). On the contrary, small scale trigeneration systems based on Thermally activated Heat Pump (THP) show low efficiency, with respect to conventional systems. This is due to the low COP of small scale cooling devices which is the reason why these systems require further improvements to be able to compete with traditional one. (C) 2012 Elsevier Ltd. All rights reserved.