CAPITULO 3 EVALUACIÓN DE LA ARQUITECTURA PROPUESTA
3.3 Evaluación de arquitecturas de software
By analyzing the shape of the grand composite curve in Figure 3.12, the required temperature levels for the hot and cold utilities can be identified. The hot utility demand does not exceed a temperature level of 100 °C. The cooling requirement is basically required at two different temperature ranges:
first, waste heat at almost 60 °C is considered as a cooling requirement since all effluents have to leave the system at the ambient temperature and the second cooling demand corresponds to refrigeration demands of the process.
The basic methodology of Section 3.4 will be applied for different scenarios of the dairy in the French context. The utility choice and the operating conditions of the heat pump for each scenario are selected graphically by analyzing the grand composite curve of Figure 3.12.
Case 0 The current situation for the given period is used as reference.
Case 1 Maximum heat recovery is calculated and the on-site available refrigeration cycle and conventional boiler are integrated.
Case 2 On-site available utilities like in Case 1 are integrated. The evaporation temperatures of the refrigeration cycle are adapted to the process cooling demand.
Case 3 As Case 2 but additionally a co-generation engine is integrated.
Case 4 As Case 2 but additionally, a heat pump is integrated.
Case 5 As Case 4 but temperature levels of refrigeration cycle and heat pump are adapted.
Case 6 Heat pump and refrigeration cycles with adapted temperature levels, co-generation and other on site available utilities (e.g. steam boiler) are integrated simultaneously.
The integrated utility curves and detailed results for each case are given in Appendix B.1. The results of all cases are summarized in Table 3.3 and the comparison of operating costs and energy savings is illustrated in Figure 3.14.
Table 3.3: Results of the dairy (Cases 1-6)
Unit Case 0 Case 1 Case 2 Case 3 Case 4 Case 5 Case 6
OpC [ke/year] 269 199 195 186 132 128 118
Ef [MWh/year] 6164 4404 4563 8416 2772 2562 4671
Eel [MWh/year] 443 429 252 -2900 382 439 -1308
Qcw [MWh/year] n.a. 2283 2249 2311 767 635 645
MCO2 [t/year] 1286 929 945 1433 595 558 823
Ep [GJ/year] 32954 24881 23504 3683 16979 16703 5595
InvC [ke] 316 326 1517 486 489 1291
P B [year] 4.5 4.4 18.3 3.6 3.5 8.6
AP [ke/year] 44 48 -39 98 102 47
The operating costs, the fuel, electricity and cooling water consumption as well as CO2 emissions and primary energy consumption are compared to the current case (Case 0). For France, following values are considered: the fuel price of 0.039 e/kWh, the electricity price of 0.062 e/kWhel, and the electricity mix with 0.092 kg/kWhelof CO2 emissions and 11.788 MJ/kWhelof primary energy (see Table 2.3).
The investment costs and payback period are evaluated by considering the cost of new heat exchang-ers (Equation (3.29)) and when necessary heat pump and co-generation units (Equations (3.26) and (3.27) respectively). For calculating the annualized profit, the interest rate is supposed to be 5%
and the life time of new installations is considered to be 20 years. Already on site available heat exchangers are not accounted. It is important to note that the payback period is only evaluated for the savings of a specific time slice. The new installed heat pump or co-generation units could also be useful in other time slices and the payback time can be decreased.
56 CHAPTER 3. PROCESS AND HEAT PUMP INTEGRATION
0 1 2 3 4 5 6 7
−50 0 50 100 150 200
Case
Savings [%]
Saving potentials higher than 200% are not displayed on this graph
CostFuel Electricity CO2 emissions Primary energy
Figure 3.14: Comparison of savings for Case 1 to 6
Heat recovery between hot and cold streams is important and leads to saving potential of about 25% of operating costs, fuel consumption, CO2 emissions and primary energy consumption. The saving potential for the electricity consumption is rather low.
Compared to Case 1, in Case 2 only the evaporation temperature of the refrigeration cycle is adapted to the process demand and a higher saving potential in electricity consumption can be observed (reduction of 41% of electricity consumption for refrigeration). To ensure the practical feasibility, the ∆Tmin between the process and the refrigeration includes the temperature difference of the heat exchangers and when necessary of the distribution network.
Case 3 considers the integration of a co-generation engine. The fuel consumption is increased but the electricity balance shows the possibility of exporting electricity to the grid. The global operating costs are not drastically smaller than in Case 2. Regarding the payback period and the annualized profit, Case 3 is not profitable.
The electricity consumption of Case 4 is increased compared to Case 2 due to the integration of a heat pump unit. The operating costs and the fuel consumption are significantly decreased, which results in reduction of fuel consumption of 39% and CO2 emissions of 37%. According to these results the COP can be estimated to 12.6 for a heat pump temperature difference of 20 °C. Graphically, the utility integration can be evaluated by the integrated composite curves (Mar´echal and Kalitventzeff, 1996) in Figure 3.15. It can be seen that more heat than predicted
with conventional pinch analysis is upgraded by the heat pump. Thanks to the combined utility integration, the condensation of the refrigeration cycle, enters into the self-sufficient pocket and heats up cold streams at low temperature. Consequently, it enables the heat pump to upgrade more heat. Available wast heat without exploiting the self-sufficient zone
Refrigeration evaporation Heat pump condensation
Cooling water
Figure 3.15: Integrated Carnot composite curves of Case 4 with refrigeration cycle, heat pump and boiler
Figure 3.16: Integrated Carnot composite curves of Case 5 with adapted refrigeration cycle, heat pump and boiler
Case 5 improves the heat pump integration by adapting the temperature levels of the refrigeration.
Even more heat can be upgraded by the heat pump, since the condensation of the refrigeration cycle enters at higher temperature into the self-sufficient zone and satisfies a bigger part of its hot demand (see Figure 3.16). The results can also be presented on the following two figures:
58 CHAPTER 3. PROCESS AND HEAT PUMP INTEGRATION
Figure 3.17 shows the integrated Carnot hot and cold composite curves and Figure 3.18 shows the integrated Carnot grand composite curve. This case offers the most promising payback times as well as the best energy savings.
0 2000 4000 6000 8000 10000 12000
−0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Heat Load [kW]
Carnot Factor 1−Ta/T[−]
Cold streams Hot streams
Figure 3.17: Integrated Carnot hot and cold composite curves of Case 5
0 200 400 600 800 1000 1200 1400
−0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Heat Load [kW]
Carnot Factor 1−Ta/T[−]
All streams
Figure 3.18: Integrated Carnot grand composite curve of Case 5
The last case (Case 6) where heat pump and co-generation units are integrated, has the lowest operating costs, but investment costs are relatively high due to the co-generation engine.
Table 3.4 compares the sizes and related investment costs of heat pumps, refrigeration cycles and co-generation engines. It has to be mentioned that the size of the different units can vary considerably for different scenarios. Especially, the size of the co-generation unit is divided by a factor 1.8 when it is integrated together with a heat pump.
Unit Power [kW] InvC [ke]
Case 1 ref 162
-Case 2 ref 95
-Case 3 ref 95
-cog -3152 1222
Case 4 ref 95
-hp 49 124
Case 5 ref 112
-hp 53 134
Case 6
ref 112
-hp 54 136
cog -1750 821