Table 5.22: Technical data of absorption refrigerators (Source: OEKOTOP)
Heating medium
Max. ambient temperature
Heating temperature
Cooling temperature
- refrigerator
- freezer
Efficiency
Gas consumption
Energy-
consumption indices
gas, kerosene, electricity
40ºC
100-150 °C
5 - 10 °C
down to approx. -12 °C
1.5 - 4.0% of the thermal input
a) calculable via the desired refrigeration capacity
b) conversion of factory data via power input
1-4 W/l useful volume
0.3-0.81 biogas/l useful volume X h
5.5.4 Biogas-fueled engines
Basic considerations
The following types of engines are, in principle, well-suited for operating on biogas:
- Four-stroke diesel engines: A diesel engine draws in air and compresses it at a ratio of 17:
1 under a pressure of approximately 30-40 bar and a temperature of about 700 °C. The
injected fuel charge ignites itself. Power output is controlled by varying the injected amount
of fuel, i.e. the air intake remains constant (so-called mixture control).
- Four-stroke spark-ignition engines: A spark-ignition engine (gasoline engine) draws in a
mixture of fuel (gasoline or gas) and the required amount of combustion air. The charge is
ignited by a spark plug at a comparably low compression ratio of between 8: 1 and 12: 1.
Power control is effected by varying the mixture intake via a throttle (so-called charge
control).
Four-stroke diesel and spark-ignition engines are available in standard versions with power ratings
ranging from 1 kW to more than 100 kW. Less suitable for biogas fueling are:
- loop-scavenging 2-stroke engines in which lubrication is achieved by adding oil to the liquid
fuel, and
- large, slow-running (less than 1000 r.p.m.) engines that are not built in large series, since
they are accordingly expensive and require complicated control equipment.
Biogas engines are generally suitable for powering vehicles like tractors and light-duty trucks
(pickups, vans). The fuel is contained in 200-bar steel cylinders (e.g. welding-gas cylinders). The
technical, safety, instrumentational and energetic cost of gas compression, storage and filing is
substantial enough to hinder large-scale application. Consequently, only stationary engines are
discussed below.
Essential terms and definitions
Knowledge of the following terms pertaining to internal combustion engines is requisite to
understanding the context:
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