Heat Engines Flashcards
What’s the efficiency equation for an externally heated/cooled heat engine?
eff = 1/- (tc/th)^0.5
What is the limiting value of a heat engine’s efficiency?
it is set by the Carnot cycle limit: essentially the higher the temperature difference the higher the efficiency
why is the same amount of power more valuable the same amount power from thermal heat?
Electrical power can be turned into heat with 100% efficiency, the reciprocal is not true. Heat can be converted to mechanical power via a heat engine with, at best Carnot cycle efficiency which will be well below 100%. Hence, electrical power tends to be regarded as more valuable
explain why combined heat and power, CHP, appears to be so attractive
CHP look very attractive because it offers the potential of using ‘waste’ heat in a valuable way. Successful CHP involves using high temperature thermal energy to run heat engines; the waste heat from the heat engine is then used for ‘low grade’ thermal tasks such as space heating, raising hot water etc.
HP is inherently attractive, but becomes even more so if the price of energy increases (increased savings), and if society is worried about climate change and carbon foot print, where the use of CHP means that there will be less fossil fuel burn than there would have been without it
Give three reasons why CHP look attractive
1, Reduced overall energy consumption.
2 Reduce C footprint for same levels of functionality
3 * Better use of available entropy; using high temperature energy for power and low temperature for heat*
Give two disadvantages of CHP
1 Need for new infrastructure (to distribute low-grade heat)
2 Need to have loads/demands which are consistent with the CHP energy split. If the mismatch is large, CHP looks less attractive, if the mismatch is very large, CHP can become economically unviable.
Explain the underlying physics which lead to the carnot cycle efficinecy equation
- the efficiency is defined as the ratio of mechanical power out to thermal power into the heat engine.
- For a perfect (ideal) engine, the mechanical output is equal to the net thermal power consumed (= thermal power in minus thermal power out)
- Gas laws are then used to demonstrate that ratios of heat inputs and outputs are uniquely related to temperature ratios, for a very special heat cycle (the Carnot cycle) in which heat is supplied at a hot isothermal temperature Th and removed at a cold isothermal temperature Tc.
- The remaining parts of the cycle are carried out adiabatically.
Why are internal combustions engines more efficient than external combustion engines?
- from equation, higher temp higher efficiency
- The hottest temperatures possible are during and in the combustion zones.
- Hence the highest efficiency are likely to be achieved in enegines that have their working fluid approaching the combustion temperatures. (This is only possible in internal combustion machines. )
- External combustion engines have operating working gas temperatures lower that combustion temperatures as a driving temperature difference is needed (and lost) to drive heat into the working fluid.
why for an external combusition engine is the power zero at maximum efficiency?
- It is possible to get an external combustion engine to have an efficiency approaching that of a Stirling engine, but this requires the hot temperature, Th to be the same on the outside and in the working fluid.
- heat flow would fall to zero and the power output would also drop to zero.
- get power from the engine, there needs to be a driving temperature difference between the hot temperature and the working fluid, hence the efficiency drops, but the power output increases .
What would be the best heat engine to use with thermal solar? (case 1 - high temp) and case 2 - lower temp
If solar thermal were to be used to generate (very) high temperature (say greater than 500C, then something like an external heat engine (Stirling) may be attractive, as this would reduce the working fluid inventory and eliminate any need for secondary working fluids. I
f the ‘collection’ temperature is lower, then, it may be more appropriate to heat a phase change fluid and go through a vapour turbine. This could be as simple as boiling water and going through a steam turbine, or if the temperatures are lowish, a lower boiling point fluid could be used. In this case, there may be a need to use an intermediate heat transferring fluid.
why is the maximum operating temperature of an externally working heat engine is lower than the temperature of the source?
- to ensure a flow of heat there must be a heat difference
- also need to ensure there’s a heat difference between sink and source (carnot efficiency)
define cogeneration
Using the ‘waste’ heat from power generation as a valuable input to a process requiring heat.
Optimised co-generation uses thermal energy in a manner which produces the optimum mix of
power and heat to satisfy/meet the customer/society requirements.
What is a sterling engine?
A Stirling Engine, SE, is an externally heated engine, hence, provided the temperature is high enough, the quality of the fuel is not (or certainly less) important. The basic engine is a closed cycle gas engine where gas is heated and allowed to expand against a piston (thus providing work), the gas is then cooled and compressed by a piston ( a smaller amount of work being done on the gas).
The net work balance is such that the engine is a work generator. The clever thing about SE is the rather than simply heating and cooling the working fluid, the fluid passes over a regenerator where it donates heat (when it isabout to be cooled) and gains heat before being heated.
A perfect regenerator makes the SE Carnot cycle efficient.
It appears attractive because:
a. External combustion…happy with dirty fuels or waste heat
b. Thermodynamically as efficient as possible,
c. Good at steady output
However:
a. Tends to have low power density,
b. Difficult to lubricate
c. Limited high temperatures (compared with internal combustion engines)
d. Poor at rapid load following
