The cogeneration is a combined production of heat and electricity, suitable for fossil fuel or biofuel (biomass) combustion systems. Cogeneration is the best solution for energy saving and environmental preservation [18] [19]. Cogeneration is a well-advanced technology that has existed for more than a century. At the end of the 19th century several manufacturing plants adopted this technology. Cogeneration requires a heat exchanger to absorb and recover exhaust heat [17] [20]. Biomass cogeneration is considered an effective alternative to reduce greenhouse gas emissions due to their low CO₂ emission [21] [22] [23]. Many researches have been conducted in recent years to improve the economic and environmental efficiency and effectiveness of biomass cogeneration systems [17] [19] - [28]. Biomass cogeneration systems are becoming increasingly popular [29]. Several cogeneration technology and systems have been developed in recent years, some of which are suitable for large power plants and other for medium power and micro-cogeneration.

The operating principle is in line with the classic Clausius-Rankin process (Figure 2). High temperature, high pressure steam generated in the boiler and then enters the steam turbine. In the steam turbine, the thermal energy of the steam is converted into mechanical work. The low-pressure steam leaving the turbine enters the condenser housing and condenses on the condenser tubes. The condensate is transported by the water supply system to the boiler, where it is reused in a new cycle [30] [31] [32].

The process of producing electricity and heat from steam includes the following components: a biomass combustion system (combustion chamber), a steam system (boiler plus distribution systems), a steam turbine, an electricity generator and the heat distribution system for heating from the condenser. At present, electricity and heat generation in biomass power plants with a steam cycle remains the most developed technology, adapted to high temperatures and high power; however, this technology is not suitable for cogeneration systems with a power of less than 100 kW compared to its low electrical efficiency and high investment costs [33] [34].

Biomass cogeneration plants generally use grid combustion systems with a thermal combustion capacity of 20 to 30 MW. In the case where chemically untreated wood biomass is used, the steam temperature reaches 540˚C. The achievable annual electrical efficiency depends on the steam parameters (temperature and pressure) and the temperature level required for the heating process. Annual electricity efficiencies generally range from 18% to 30% for biomass cogeneration plants between 2 and 25 MW [30]. Below the advantages of the use of steam cycle:

· The use of water as a heat transfer fluid has great advantages, such as its high availability, non-toxic, non-flammable, chemical stability, low viscosity (less friction losses);

· Thermal efficiency greater than 30%;

· Low pump consumption.

Since the 1980s, the ORC market has grown exponentially [34]. ORC applications have generated a lot of economic and environmental interest, because of which much work has been done on ORC systems and working fluids that can be found in the literature [35] - [40].

ORC technology (Figure 3) has reached a very high degree of maturity for biomass applications; it only requires a sufficient source of heat. The ORC system can be integrated into any industrial facility equipped with a low temperature heating system to recover waste energy in the form of heat and convert it into electricity. Electricity produced by biomass ORC systems is considered carbon neutral, thus improving a company’s environmental profile and promoting the transformation of the forest sector towards the use of more environmentally friendly energy sources [39] [41].

Instead of water, the Rankine organic cycle uses an organic fluid with favorable thermodynamic properties as a heat transfer fluid. The evaporation temperature of organic fluids is lower than the evaporation temperature of water, which results in higher efficiency in cogeneration installations with an ORC cycle.

The ORC has two circuits, one for thermal oil and the other for organic fluid. The heat released by the combustion of biomass is transmitted through an oil cycle by an exchanger to the organic fluid, which evaporates at high temperature and high pressure. The ORC system consists of four main components, namely a pump, an evaporator, a turbine and a condenser. The superheated organic steam is expanded in a turbine and then condensed in a condenser and returned to the circulation pump to start a new cycle. The condenser can act as a heat exchanger for sending heat remotely at low temperatures (e.g. district heating [42]). The condensed organic liquid is pumped through the regenerator to the evaporator [43]. ORC technology is suitable for medium power [19]. Heat is generally supplied at a temperature of about 300˚C and condensation occurs at about 90˚C [44]. There are more than 50 biomass cogeneration plants that have adopted CRO technology with a capacity greater than 5 MWe and have approved the technical and economic feasibility of this technology on a medium scale (200 - 2000 kW) [45]. The thermal efficiency of ORCs at high temperatures does not exceed 24% [34].

The organic fluids used in these systems are dry and do not require overheating, they are not corrosive or erosive; they evaporate at low and medium temperatures [47] [48]. When temperatures exceed 500˚C, the organic liquid degrades and turns into small particles [43].

Table 1 shows the largest suppliers and manufacturers of ORC technologies in a wide range of power and temperature levels.

According to [34] [41] [48] and [49] advantages of ORC installations are:

· Long service life due to the characteristics of the working fluid;

· Less complex installation with a high efficiency cycle;

· More economical than a water steam turbine in terms of investment and maintenance costs;

· The isentropic efficiency of a turbine varies with its power scale and design;

· No water treatment system is required;

· The system pressure is low, which makes the installation safer;

· No need for fluid control;

· Efficient solution for low temperature installations.

In a Stirling engine (Figure 4), the regenerator is an internal heat exchanger and temporary heat store placed between the hot and cold spaces such that the working fluid passes through it first in one direction then the other, taking heat from the fluid in one direction, and returning it in the other. It can be as simple as metal mesh or foam, and benefits from high surface area, high heat capacity,

low conductivity and low flow friction. The primary effect of regeneration in a Stirling engine is to increase the thermal efficiency by “recycling” internal heat that would otherwise pass through the engine irreversibly. As a secondary effect, increased thermal efficiency yields a higher power output from a given set of hot and cold end heat exchangers. These usually limit the engine’s heat throughput. In practice, this additional power may not be fully realized as the additional “dead space” (unwept volume) and pumping loss inherent in practical regenerators reduces the potential efficiency gains from regeneration [50].

The Stirling engine is based on a closed cycle where the working fluid which is usually hydrogen or helium is compressed in the cold cylinder and expanded in the hot cylinder [50] [51] [52] [53]. The design challenge for a Stirling engine regenerator is to provide sufficient heat transfer capacity without introducing additional internal volume (“dead space”) or flow resistance. These inherent design conflicts are one of many factors that limit the efficiency of practical Stirling engines. A typical design is a stack of fine metal wire meshes, with low porosity to reduce dead space, and with the wire axes perpendicular to the gas flow to reduce conduction in that direction and to maximize convective heat transfer.

Stirling engines are mainly used in residential micro CHP, i.e. the local production of electrical and thermal energy in residential buildings [55] [56]. Sterling engines are the most commonly cited technology in biomass micro cogeneration and the greatest advantage of this engine is its ability to burn any type of biomass [55] [56] [57] [58] [59]. The use of Stirling engines as electricity generators in domestic cogeneration systems has attracted a lot of interest in recent years due to the ever-increasing price of energy [60] [61] [62]. Figure 5 shows the diagram of the operating principle of cogeneration with the Stirling engine, while Table 2 illustrates the largest manufacturers and suppliers of Stirling engines.

According to [52] - [64] advantages Stirling installations are:

· Very long service life;

· The motor runs quietly, which makes it ideal for use in tertiary and residential sectors;

· The torque produced is very regular, which maintains the good condition of the parts;

· The Stirling engine does not require periodic oil change because the lubricant is not in contact with combustion residues (external combustion);

· Low maintenance needed;

· Low GHG emission, which favors its carbon footprint.

The largest suppliers and manufacturers of cogeneration systems with Stirling engines are listed in Table 3.

The Ericsson engine is a member of the hot air or external combustion engine family. It is a reciprocating engine that operates with the open Joules cycle, it is composed of two cylinders (compression cylinder and expansion), a heat exchanger that allows the adjustment of the inlet and valve temperature levels at the inlet and outlet of the cylinders [65] [66]. The working fluid, which is air, enters the compression cylinder at atmospheric pressure and ambient temperature, is compressed and then sent to the heat exchanger, its temper increases and then enters the expansion cylinder where it expands so that it is finally discharged outside the engine [67] (Figure 6).

Numerous studies found in the literature have shown that the Ericsson engine has high performance for low power levels such domestic micro-cogeneration with renewable energies such as solar and biomass, mainly for micro-cogeneration with solar concentrators [66] [67] [68] [69] [70].

Advantages of the Ericsson engine according to [70] and [71] are:

· Low maintenance;

· Silence operation;

· Simple technology;

· Low costs and better performance;

· Valves can contribute to engine control and command;

· Low GHG emissions.