A Comparative Study of Open and Closed Heat-Engines for Small-Scale CHP Applications
Abstract
:1. Introduction
2. Externally Heated Versus Internally Heated Engine Cycles
2.1. Comparison of Thermal Efficiency
2.2. The Recuperated Constant Pressure Heat Engine Cycle
3. Comparison of the Externally Heated RJC (Recuperated Joule Cycle) and Stirling Cycle
- (1)
- Both the RJC and Stirling cycles are internally reversible.
- (2)
- Both cycles are heated externally via identical heat exchangers using hot combustion flue-gas: a variable temperature heat source.
- (3)
- In both engines the combustion gases are assumed to enter the high-temperature heat exchanger at the same temperature (TH1) and leave at the same temperature (TH2).
- (4)
- Both cycles are assumed to absorb heat at the same rate (J/s).
- (5)
- Either ∆Tm,H,RJC = ∆Tm,H,SC or ATDH,RJC = ATDH,RC (both possibilities were investigated).
- (6)
- For the RJC the heat capacity rate, (CH), of its working fluid equals that of the combustion gases.
- (7)
- The working fluid within both cycles is dry-air, which is assumed to be a perfect gas.
- (8)
- (9)
- The temperature of the air leaving the Stirling cycle’s low-temperature heat exchanger equals that leaving the recuperator of the RJC: T6 in Figure 2a equals TL2 in Figure 5. This is thought to be a reasonable assumption if the waste heat from both cycles is to be utilized for heating purposes because it would mean its temperature in both cases would be equal.
- (10)
- For the purpose of analysis the ambient air temperature is assumed to be 300 K.
3.1. Comparison of Efficiency Based on Equal ∆Tm,H Values
3.2. Comparison of Efficiency Based on Equal ATDH Values
3.3. Some Results and Discussion
4. Conclusions
Author Contributions
Conflicts of Interest
Nomenclature
A | heat transfer area (m2) |
ATD | approach temperature difference (K) |
C | heat capacity rate = mCp (W/K) |
CHP | combined heat and power |
Cp | specific heat capacity at constant pressure (J/kg·K) |
m | mass flow (kg/s) |
NTU | number of heat transfer units |
Pr | RJC cycle pressure ratio (−) |
Qinput | heat input rate (W) |
RJC | recuperated Joule cycle |
s | specific entropy (J/kg·K) |
T | temperature (K) |
Tc | isothermal compression temperature (K) |
Te | isothermal expansion temperature (K) |
ΔTm | area-weighted (or log-mean) temperature difference (oC) |
U | overall heat transfer coefficient (W/m2·K) |
Greek letters | |
α | isentropic compression temperature ratio (−) |
ε | heat exchange effectiveness (−) |
θ | cycle temperature ratio, TH1/TL1 (−) |
θ* | cycle temperature ratio for an internally reversible engine |
isentropic compression efficiency (−) | |
isentropic expansion efficiency (−) | |
thermodynamic efficiency (−) | |
Subscripts | |
Amb | ambient air |
SC | Stirling cycle |
fg | flue-gas |
H | heat source temperature |
H1 | heat source inlet temperature |
L | heat sink temperature |
L1 | heat sink inlet temperature |
opt | optimum value |
r | recuperator |
RJC | recuperated Joule cycle |
SC | Stirling cycle |
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Manufacturer | System Name | Engine Cycle | Output | Thermo Efficiency | Reference |
---|---|---|---|---|---|
asjaGen | TOTEM 10 | Otto | 10 kW | 30% | [10] |
Helec Ltd | Energimizer | Otto | 7.5 kW | 25% | [11] |
Helec Ltd | Powerbox 7500QSE | Stirling | 7.5 kW | 18% | [12] |
Baxi Ltd | Ecogen | Stirling | 1 kW | 13% | [12,13,14] |
State-Points | Process |
---|---|
1–2 | Compression of ambient air |
2–3 | Recuperative heating |
3–4 | External heating |
4–5 | Expansion |
5–6 | Recuperative cooling |
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Eames, I.W.; Evans, K.; Pickering, S. A Comparative Study of Open and Closed Heat-Engines for Small-Scale CHP Applications. Energies 2016, 9, 130. https://doi.org/10.3390/en9030130
Eames IW, Evans K, Pickering S. A Comparative Study of Open and Closed Heat-Engines for Small-Scale CHP Applications. Energies. 2016; 9(3):130. https://doi.org/10.3390/en9030130
Chicago/Turabian StyleEames, Ian W., Kieran Evans, and Stephen Pickering. 2016. "A Comparative Study of Open and Closed Heat-Engines for Small-Scale CHP Applications" Energies 9, no. 3: 130. https://doi.org/10.3390/en9030130