Utilization of low-value heat in a supercharged thermal engine

A power plant comprises a combustion engine having a supercharger for receiving low pressure feed air and delivering compressed feed air to the engine, a feed air heater for transferring low-temperature thermal energy from a heat source to the low pressure feed air, and a feed air cooler for recovering higher-temperature thermal energy from the compressed feed air.

BACKGROUND OF THE INVENTION 
The invention relates to an arrangement for utilizing low-value heat for 
improving the total efficiency rate of a supercharged thermal engine, such 
as a diesel engine. The invention is particularly applicable to a power 
plant. The term "power plant" as used in this specification means an 
arrangement which comprises a combustion engine that produces mechanical 
energy at a substantially constant power level and is connected to drive a 
secondary machine which converts the mechanical energy produced by the 
combustion engine to another form. There are several applications for such 
a power plant. For example, in an electricity generating power plant, the 
secondary machine is an electricity generator. In a marine propulsion 
power plant, the combustion engine is the ship's main propulsion engine 
and the secondary machine may be the ship's propulsion screw or an 
electric generator that drives an electric motor which in turn drives the 
propulsion screw. The operation of a power plant releases substantial 
quantities of thermal energy. Because the combustion engine produces 
mechanical energy at a substantially constant power level, a high degree 
of optimization of the operation of the engine, particularly in recovery 
of thermal energy, can be achieved. 
Utilizing heat flows of a combustion engine for preheating or heating 
purposes or for driving auxiliary equipment is known. In such application 
it is of advantage to recover heat from high-temperature heat flows, such 
as exhaust gases or supercharged feed air. U.S. Pat. No. 4,182,127 
discloses the utilization of several different heat sources. Finnish 
Patent Application No. 933126 (corresponding to U.S. patent application 
No. 08/566,128) discloses utilization of heat recovered from the hot 
coolant of an engine, from exhaust gases and from supercharged feed air. 
The latter document mentions that further, not specified cooling of the 
supercharged air supplied to a combustion engine is necessary for lowering 
its temperature, but nothing is said about any utilization of the heat 
recovered in this cooling process. This is because low-temperature heat of 
this kind is normally considered to be "low-value heat," that is heat that 
is not worth being utilized and therefore is left unutilized. This is 
harmful to the efficiency rate of the engine, because the temperature of 
the feed air remains disadvantageously high. 
SUMMARY OF THE INVENTION 
An object of the invention is to utilize available flows of low-value heat 
for improving the total efficiency rate of a combustion engine plant. 
In accordance with a first aspect of the invention there is provided a 
power plant comprising a combustion engine having a supercharger for 
receiving low pressure feed air and delivering compressed feed air to the 
engine, a feed air heater for transferring low-temperature thermal energy 
from a heat source to the low pressure feed air, and a feed air cooler 
means for recovering higher-temperature thermal energy from the compressed 
feed air. 
In accordance with a second aspect of the invention there is provided a 
method of operating a power plant that includes a combustion engine having 
a supercharger for receiving low pressure feed air and supplying 
compressed feed air to the engine, said method comprising transferring 
low-temperature thermal energy to the low pressure feed air, and 
recovering higher-temperature thermal energy from the compressed feed air. 
Because heat is transferred to the feed air of a supercharged engine prior 
to leading the feed air to a turbocharger, or the like, the temperature of 
the air downstream of the charger is higher than without said heat 
transfer. Therefore, the heat of the supercharger feed air is more useful 
for utilization. Preheating of engine feed air according to the invention 
thus increases the amount of utilizable high-temperature heat. In this 
specification, the term high-temperature means a temperature that is 
higher than the temperature of the low-value heat referred to and suitable 
for known heat utilization purposes. Heat used for feed air preheating may 
be any low-value or waste heat, for example heat recovered from the same 
feed air downstream of the turbocharger. Its use for the purpose mentioned 
can only be of advantage and increase the total efficiency rate of the 
engine, because it decreases the temperature of the air fed into the 
engine to an extent that without applying the invention would not be at 
hand. 
Improving the total efficiency rate of a power plant according to the 
invention, may be influenced by using heat from a medium flow cooling a 
machine powered by the prime mover of the plant, for example an electric 
generator. In particular in high power applications these medium flows 
contain substantial amounts of heat. Also, heat recovered from any cooling 
system of a combustion engine plant may be utilized for the same purpose. 
Heat exchangers are normally used for transferring heat from one fluid 
flow to another. The most efficient heat transfer is provided by heat 
exchangers working in the contra-flow mode. 
The best results with respect to the total efficiency rate are normally 
achieved when several different heat sources are jointly used for 
preheating the feed air. 
The invention is in particular suitable for improving the total efficiency 
rate of a power plant in which the combustion engine is a reciprocating 
internal combustion engine, for example a diesel engine. The diesel engine 
may be conventional, it may be an engine using heavy oil as fuel or a gas 
engine, possibly with spark ignition. It is of advantage that the thermal 
engine is quite big, having a mechanical power output of at least 0.5 MW, 
preferably at least 5 MW. 
When, in an arrangement according to the invention, high-temperature 
thermal energy is taken from compressed feed air, the aim should be that 
only a small amount of residual thermal energy is left. It is recommended 
that the temperature of the residual thermal energy is at the most 
85.degree. C., preferably at the most 70.degree. C. 
High-temperature thermal energy recovered from the compressed feed air may 
advantageously be utilized in a district heating system or for other 
heating purposes. If the engine fuel is heavy oil, heat is also needed for 
fuel preheating. Another way of utilizing high-temperature thermal energy 
is to convert it to electricity by means of a steam turbine. 
Normally, atmospheric air is used as feed air. Hence, the air temperature 
may vary considerably due to atmospheric conditions. Utilizing low-value 
heat is only possible when its temperature is higher than the temperature 
of the ambient air. In practice this means that the utilizable low-value 
heat temperature should be higher than 40.degree. C.

DETAILED DESCRIPTION 
In the drawings, 1 indicates a diesel engine provided with a turbocharger 
comprising a compressor 2b driven by an exhaust gas turbine 2a. The 
turbocharger compresses the engine's feed air received through a pipe 3. 
Between the compressor 2b and the engine 1 the feed air passes through a 
pipe 4 to a heat exchanger 5, which may include one or several 
heat-exchanging stages and, preferably works in the contra-flow mode, as 
shown by arrows 16. The heat exchanger 5 recovers for utilization 
high-temperature thermal energy from the feed air that has been warmed up 
considerably due to the pressure rise provided by the compressor 2b. From 
the heat exchanger 5, the feed air is led to a cooler 6, which also works 
in the contra-flow mode. The cooler 6 thus delivers cooled supercharged 
feed air with a temperature of about 50.degree. C. to the diesel engine 1. 
The engine 1 drives a generator 7 for production of electricity. Generally 
used heat recovery circuits for utilizing heat from the exhaust gases of 
the engine are not shown in the drawing. 
Liquid containing pipes 8 and 9 connect the cooler 6 to a heat exchanger 10 
working in the contra-flow mode for transferring low-value heat from the 
cooler 6 to the feed air to be delivered to the compressor 2b, thereby 
increasing the temperature of the air. A circulation pump 11 provides a 
desired liquid flow in the pipes 8 and 9. The temperature of the 
unprocessed feed air may vary significantly. If the engine operates in 
arctic conditions, the feed air temperature is typically the same or 
almost the same as the temperature of the ambient air. Because of freezing 
or humidity problems, it is sometimes necessary to preheat the air or give 
it some other treatment. In a warm climate, the temperature of the feed 
air may be +30.degree. C. or even more. The temperature of the liquid 
contained in the pipe 9 is normally 20.degree.-30.degree. C., but it may 
vary considerably in different applications and under different 
atmospheric conditions. In the cooler 6, the temperature of the liquid 
coming in from the pipe 9 usually rises by 2.degree.-20.degree. C., but it 
may rise even more, especially, if the flow rate is low and/or if the 
cooler 6 works very efficiently as a heat exchanger. 
FIG. 1 also shows a liquid cooler 14 connected to the cooling system of the 
generator 7. This cooler is connectable through pipes 12 and 13 to the 
described heat utilization arrangement. The coolers 14 and 6 may be 
connected in series or in parallel. By calculations it is possible to 
analyze which connection mode is more advantageous in each case. If 
necessary, a circulation pump 15 may be employed for ensuring the liquid 
flow through the heat exchanger 14 and for adapting the flow rate through 
the coolers 14 and 6 to a suitable value. 
It is feasible to use, for the purpose described, any available low-value 
heat flow either alone or jointly with some other heat flow for heating 
the feed air upstream of the supercharger, thereby improving, according to 
the invention, the total efficiency rate of the arrangement. 
In FIG. 2, the upper curve 17 shows the temperature of the feed air of a 
diesel engine in an arrangement according to the invention and the lower 
curve 18 shows the corresponding temperatures when the invention is not 
applied. The section 10' refers to the function of the heat exchanger 10, 
the section 2b' to the function of the turbocharger 2b, the section 5' to 
the function of the heat exchanger 5 and the section 6' to the function of 
the cooler 6. T.sub.1 refers to the temperature of the feed air upstream 
of the heat exchanger 10. The temperature T.sub.1 is normally less than 
300 C, and even significantly less than 0.degree. C. in arctic conditions. 
In an arrangement according to the invention, the feed air is warmed up in 
the heat exchanger 10 to the temperature T.sub.2, which is about 
40.degree. C., but may also be higher. Without an arrangement according to 
the invention, the feed air would enter the compressor 2b at the 
temperature T.sub.1 and would reach the temperature T.sub.3, which usually 
is 200.degree.-230.degree. C., downstream of the compressor. When the 
invention is applied, the air is warmed up by the compressor 2b from the 
temperature T.sub.2 to the temperature T.sub.4, which is significantly 
higher than T.sub.3 and normally above 240.degree. C. In the heat 
exchanger 5, high-temperature heat is recovered for utilization. 
The temperature T.sub.6, usually 80.degree.-90.degree. C., is the 
temperature of the compressed feed air downstream of the heat exchanger 5, 
both when applying and not applying the invention. This means that, in an 
arrangement according to the invention, one may recover, in the heat 
exchanger 5, significantly more high-temperature heat for utilization than 
when not applying the invention. In the cooler 6, the temperature of the 
compressed feed air decreases to the temperature T.sub.5, which is 
approximately the same when applying the invention and when not. Normally, 
the temperature T.sub.6 is at the most about 50.degree. C. It should be 
noted that FIG. 2 only refers to one application of the invention. 
Adjustment of the functional effectiveness of the cooler 6 and/or the heat 
exchanger 5 might give a more favorable result. The high-temperature heat 
recovered from the heat exchanger 5 is suitable for district heating, for 
preheating heavy oil, for steam generation and/or for other purposes of 
similar use. 
FIG. 3 shows a modification of FIG. 1. As shown in FIG. 3, the engine 1 has 
a cooling system through which engine coolant circulates under control of 
a pump 18. The engine's cooling system includes a heat exchanger 17 for 
recovering thermal energy from the engine coolant. The heat exchanger 
includes a heat exchanger stage 17a for transferring thermal energy to the 
liquid circulated through the heat exchanger 5. The heat exchanger 17 may 
have a second stage 17b for transferring thermal energy to another fluid 
flow. Whether the stage 17a is a lower temperature stage than the stage 
17b, as shown in FIG. 3, or a higher temperature stage, depends on which 
configuration is more favorable with regard to overall efficiency of the 
power plant. 
The invention is not limited to the embodiments shown, but several 
modifications are feasible within the scope of the attached claims.