Combined Micro-Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 68

Two types of internal combustion engines are used to generate energy:

spark ignition engines (with plugs, Otto);

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internal combustion engines ignited by compression, where the mix is brought
to a high pressure before entrance into cylinders (Diesel).

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Piston engines are available, from those coupled to small generators (0.5 kW) to
those coupled to large generators (3 MW). These use common fuels like petrol,
natural gas, diesel, and are convenient in a multitude of applications, due to their
small sizes and their low costs. In energy production, the functioning of piston
engines includes both continuous functioning and functioning at peak charges, as
spare source. Piston engines are ideal for applications where there is a substantial
need for hot water or for low pressure steam.
In the case of a CHP system used for producing electricity on a wide scale,

ef ciency is

of approximately 48 %. The heat in the burning gases is lost, together with their
exhaust into the atmosphere. Unlike this case, with a mCHP system based on
internal combustion thermal engines, the heat from the cooling water, from the
machine oil and from the exhaust circuit of burning gases is recuperated. From the
recuperated heat, low pressure steam or hot water may be obtained, which may then
be used for heating, obtaining domestic hot water and refrigerating.
The heat from the cooling circuit is able to produce hot water at 90 ° C and
represents approximately 30 % of the fuel input energy. Engines which work at
high pressure are equipped with a cooling system for very high temperatures and
can work to up to a temperature of 455

ciency is around 30 % while, during a combined exploitation cycle, ef

C. Since the temperature of the
exhaust gases must be kept above the condensation limit, only part of the heat
contained by these gases can be recuperated. Heat recuperation units are generally
designed for a temperature of 150

- 649 °

C, to avoid corrosion and condensation in
the exhaust pipes. Low pressure steam and hot water 110

- 175 °

° C is produced using the

machine fluid and from the

gases resulted from the burning process may reach approximately 70

' s heat exhaust. The heat recuperated from the cooling

80 %.
The performance characteristics of internal combustion engines are given in
Table 3 .

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Table 3 The performance characteristics of the CHP systems with internal combustion engines as
prime mover [ 10 ]
Power range (kW e ) 10 100 3,000

Power to heat ratio 0.50 0.79 0.97

Electrical ef ciency (%) 25 - 28 34 36

Total ef ciency (%) 79 78 73

Fuel Input (MMBtu/hr) 0,5 4,9 28

Engine Speed (rpm) 1,500 1,500 750

Fuel type A variety of gaseous and fluid fuels