1、A Parametric Study for Integrated Design Optimization of Low-Energy Buildingsg3g3Siir Kilkis Birol Kilkis, Ph.D. Student Member ASHRAE Fellow ASHRAE ABSTRACT Beyond a low-energy and low-exergy building concept, the mechanical system that converts energy resources to useful mechanical power and HVAC
2、functions need to be optimized for maximum efficiency with minimum energy waste and exergy destruction. This paper provides a new analytical algorithm, which optimizes the CHP, absorption chiller, heat pump, alternative energy and power systems like wind and solar, TES, PES, and HVAC terminal unit c
3、ombinations and capacities for a given building. A case study is presented for several mechanical system scenarios. Results show that lower CO2impact buildings are possible even when fossil fuels are used. INTRODUCTION New dwellings are generally equipped with split-type air conditioning units witho
4、ut considering central cooling (CC) or district cooling (DC) options. In fact, the overall CO2footprint of these units when compared to the district cooling systems need to be carefully investigated. For example, district cooling systems offer higher efficiency and more options of generating cold fr
5、om a variety of energy resources and fuels rather than simply relying on electric power 1. Moreover, district cooling systems offer a peak-load shaving option by accommodating cold storage systems, which further improve the performance of comfort cooling in general. Another advantage of CC or DC sys
6、tems is the ability to use co-generation and tri-generation systems effectively with substantial fuel savings 2-4. Such a variety of district cooling system options enriches the potential solutions for reducing CO2emissions, cooling costs, and increasing efficiency. Yet at the same time, this richne
7、ss of options brings the necessity and complexity of searching for optimum solutions in order to grasp the full advantages of district cooling. Such an optimization process must simultaneously focus on economy, environment, efficiency, and comfort with equal emphasis. The truth is that all these fou
8、r parameters are related and usually conflict with one another. The conflict is weakest when the environment and the energy efficiency with respect to both the first-law and the second-law of thermodynamics are considered. The conflict is strongest between economy and comfort parameters but they are
9、 individually satisfied when the environment and energy parameters are optimized. Thus, any optimization algorithm that covers environment and efficiency may sufficiently satisfy all parameters. In contrast, the complexity of this optimization problem often makes designers ignore this need and conti
10、nue to use conventional, unitary systems, except large buildings with central plants that are already in order. In this study, the compound CO2emission relationship from the Rational Exergy Management Model (REMM) 5 was employed for a new, comprehensive yet simplistic optimization algorithm for dist
11、rict cooling, especially in hot climates. An individual building (i) may be responsible from three-point emission sources. The first one is the on-site emission source like a boiler (the first term in Equation 1). The second source is due to exergy destruction that takes place in the building due to
12、 mismatches of the supply and demand exergy (avoidable) and if there is any, the emissions at the power plant due to the electricity demand of the mechanical system in the building (indirect) is the third source 5, 6. However, in the built environment, the collection of indirect terms of each buildi
13、ng drops from Equation 1 because they are actually embedded in other buildings avoidable terms. In an extreme case, if all buildings do not spend any heat but just electric power, then the avoidable term of the power plant takes care of all indirect terms of the buildings. Figure 1 shows how OF chan
14、ges with fuel type and g524Rifor a building, which has a boiler for heating and receives power from the grid (Base Scenario). The sensitivity of CO2emissions on the fuel type increases with the ci value. For example, the slope of the line for lignite, which has the highest civalue, is the steepest.
15、For a given fuel type CO2emissions decrease with g524Ri. This relationship is depicted in Equation 1. Siir Kilkis is a Ph.D. student at KTH, Stockholm, Sweden. Birol Kilkis is a professor at Bag250kent University and Head of Energy Engineering Graduate Program, TurLV-11-C054442 ASHRAE Transactions20
16、11. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES p
17、rior written permission.Figure 1 Compound CO2emissions with respect to g524Riand fuel, Pe= 0.5 Ph5. (1) 1- Direct 2-Avoidable 3- Indirect In calculating the compound CO2emissions, exergy destructions in the building must be accounted for at the central power plant (j). When the same building is cool
18、ed with unitary air conditioners, however, direct emissions are absent. In particular, it is this absence that misguides us into assessing unitary air-conditioners as “green.” In essence, such units have very low rational exergy management efficiency, namely g524Ri, which is below 0.1. Together with
19、 the power that they demand from central power plant with the avoidable term, they offset the absence of the first term. For a unit cooling demand (Ph= Pc = 1 kWh) and a distant natural gas-fired combined-cycle power plant (Tig75g75= 0.5, cj = 0.2 kg CO2/kWh), which supplies power to the air-conditi
20、oning unit with an average COPcof 2.5, g524Riis 0.05 and because power is supplied at a rate of Pc/COPc:, the compound CO2emission is: (2) Rather than using the unitary air-conditioning system explained above, if a tri-generation system runs on natural gas and its useful heat is utilized in a heat-o
21、perated cooling machine and domestic hot water demand, then Equation 3 applies. Here, the electric power required for such a system will be limited to the parasitic and ancillary loads (maximum 10%). Remaining power is given back to the grid. For this scenario the following base assumptions were mad
22、e: absorption chiller COPCH= 1.25, g536i = 0.56 (Heat generation efficiency) g536T= 1 (on-site power transmission), g536j = 0.35 (Power generation efficiency), g524Ri= 0.65 4. A parametric study is given in Figure 2. Here, with an increase of (g536j/g536i), the primary energy savings decrease and CO
23、2emissions increase, although the exergy efficiency increases with an increase of power generation rather than heat. Equation 3 depicts that the CO2emission responsibility of the specific building for its on-site power generation is limited to the amount that it spends on-site. (3) g11g12eTjjhRiTiji
24、ijiiPcPccCOCOCOOF g117g184g184g185g183g168g168g169g167g14g117g187g188g186g171g172g170g16g184g184g185g183g168g168g169g167g14g184g184g185g183g168g168g169g167g32g39g14g32g32g166g75g75g92g75g75g751222hRicTijiPCOPcCOOF g117g187g188g186g171g172g170g184g184g185g183g168g168g169g167g16g14g184g184g185g183g168
25、g168g169g167g32g32g166g92g75g75112/kWh CO kg 0.54kWh 105.05.2115.02.0023.058,0222 CiCOPiCOCO g16g35g166g32g117g187g188g186g171g172g170g184g185g183g168g169g167g16g14g184g185g183g168g169g167g32g166g11g12 /kWhCO kg 0.161.035.02.0165.0156.02.022g32g117g184g185g183g168g169g167g14g117g187g188g186g171g172g
26、170g16g184g185g183g168g169g167g32g32g166cHiCOPCOOF 2011 ASHRAE 443loadkWh/unit 0.03429328311demwinterg32g184g185g183g168g169g167g16g32g184g184g185g183g168g168g169g167g16g32arefTTg72Figure 2 Effect of the power to heat ratio on Primary Energy Savings (PES) and the Objective Function (OF) for a micro-
27、tri-generation system in the building 5. PARAMETRIC ALGORITHM Equation 4 is the minimization function of comfort cooling in the building stock. The optimization variables are; ci, cj, g536i, g536j, g536T, g524Ri, Ph(and or Pc), Pe. Thermal and electrical loads, namely Ph(or Pc) 0, Pe 0) are assumed
28、to be already minimized by conventional energy savings measures. Therefore, the optimization problem reduces to six variables with the following constraints: 0 0.7; nT 0.9; 1 g524Ri 0.7 (In order to qualify “exergy green system 5) g198Minimize (4) supg72g72g92demRig32(5) (6) Demand exergy for winter
29、 and summer conditions, respectively are 5, 6: Comfort Heating (7) Comfort Cooling (8) The following numerical sample solution explains these equations, where: Tf= 2000 K; Tref= 283 K Ta = 293 K (winter), 297 K (summer); To= 303 K (summer) loadkWh/unit 0.86200028311supg32g184g185g183g168g169g167g16g
30、32g184g184g185g183g168g168g169g167g16g32KTTg304frefg11g12eTjjhRiTijiiPcPccOF g117g184g184g185g183g168g168g169g167g14g117g187g188g186g171g172g170g16g184g184g185g183g168g168g169g167g14g184g184g185g183g168g168g169g167g32g75g75g92g75g75g751g184g184g185g183g168g168g169g167g16g16g184g184g185g183g168g168g1
31、69g167g16g32areforefdemTTTT11g72g184g184g185g183g168g168g169g167g16g32frefTT1supg72g184g184g185g183g168g168g169g167g16g32arefdemTT1g72444 ASHRAE Transactionsg524Riwinter = 0.034/0.86 =0.04 loadkWh/unit 0.01882972831303283111demsummerg32g184g185g183g168g169g167g16g16g184g185g183g168g169g167g16g32g184
32、g184g185g183g168g168g169g167g16g16g184g184g185g183g168g168g169g167g16g32agogTTTTg72g524Risummer= 0.0188/0.86 = 0.02 Equation 4 may be minimized by taking partial derivatives of the optimization variables in a sequence and then equating each of them to zero. If it is assumed that the same type of fue
33、l is used both in the building (i) and the power plant (j), then ci, cjare equal and the number of variables reduce from six to five. For simplicity, keeping g536Tfixed for the existing national grid, and for unit Phand Pe: (9) The solution g524Ri = 3 is not admissible because of the condition g524R
34、i g148 1. The latter condition may be satisfied if Tfapproaches to Tain heating (See Equations 6 and 7) and if Tfapproaches to Tref, in cooling (See Equations 6 and 8). This means that the lowest possible supply temperature-bearing energy (waste) sources must be utilized and coupled with low-exergy
35、buildings. If OF is maximized the building may approach to net-zero energy status when a building is both thermally and electrically connected to the grid, especially when the building uses alternative fuels. If g524Riapproaches to one then the optimization function reduces to Equation 10. The latte
36、r two conditions in Equation 10 indicate that alternative energy resources must be mobilized in order to reduce ciand cjif they are economically available in the vicinity of, or on the site. For a district cooling system, the distance between the supply and demand points for a closed circuit system
37、is given by Equation 11 3, 7, 8. OF = (Maximize g524Ri), g536i, g536j, g536Tand minimize ci and cj(10) 3.1max201000g184g185g183g168g169g167 g39g117g184g185g183g168g169g167g14g32TPaLnP 1000kW, g507T g148 30oC (11) where, In district heating (12-a) Here, Trezis the temperature of the heat-source for t
38、he heat-driven cooling machine(s). For example, if in a district cooling system, where heat is supplied to the on-site absorption machines, Trefis 283 K and Trezis 363 K, then n is 0.68 from Equation 12-a. In Equation 11 a is 0.6 km in district heating and 0.2 km in district cooling for pre-insulate
39、d thermo-plastic piping of optimum diameter. If hot water is circulated in the district and cold energy is generated on site, then g507T is 20oC. The compression cycle cooling with electric power (Chillers) was not considered, because their exergy efficiency is the lowest when the dwelling boundarie
40、s are considered, (electric power exergy g304supis about 1 kWh/1kWh). If cold is generated at the district plant and cold fluid is circulating in the district loop, g507T is about 5oC and Equation 12-a in district cooling is then given by Equation 12-b: In district cooling (12-b) In district cooling
41、, Tref is taken 273.15 K for convenience of calculations. If for example, in a district cooling system, cold water at 283 K (Trez) is supplied to the district, then, from Equation 12-b, n is 0.40: loadkWh 1per kWh 0.018872972831303283111demsummerg32g184g185g183g168g169g167g16g16g184g185g183g168g169g
42、167g16g32g184g184g185g183g168g168g169g167g16g16g184g184g185g183g168g168g169g167g16g32agogTTTTg72g524Risummer= 0.01887/1 = 0.01887 g11g120211g32g187g188g186g171g172g170g16g184g184g185g183g168g168g169g167g14g184g184g185g183g168g168g169g167g32g119g119RiTiiicOFg92g75g75g7533.015.333116.0g184g184g184g184
43、g184g185g183g168g168g168g168g168g169g167g184g184g185g183g168g168g169g167g16g184g184g185g183g168g168g169g167g16g117g32refzrerefTTTn2.115.280116.0g16g184g184g184g184g184g185g183g168g168g168g168g168g169g167g184g184g185g183g168g168g169g167g16g184g184g185g183g168g168g169g167g16g117g32refzrerefTTTn 2011 A
44、SHRAE 445SCENARIOS A district energy system has 160 dwellings assembled in four high-rise apartment buildings. Every dwelling has identical peak cooling loads of 10 kW. g524Riis 0.05. The total peak cooling load in the district is 1100 kW for the given diversity of use patterns. The average daily op
45、erating time, which is corrected for the peak load versus base load, is four hours at peak summer load. Four scenarios for comfort cooling were considered in this study. These scenarios were compared according to the daily compound CO2emissions. Base Scenario All dwellings are equipped with, split-t
46、ype air-conditioners operated by electric power from a distant coal fired power plant. This scenario has already been discussed in earlier sections: District Energy Scenario-1 Chilled water for comfort cooling will be prepared at the district plant and then will be served to all four apartment compl
47、exes via the district closed-loop separated from the apartment complexes by flat-plate heat exchangers. Each dwelling will be served by secondary, closed-loop hydronic circuits in each apartment complex. The temperature drop between supply and return in the closed-loop cold-water district loop is 10
48、oC. A central tri-generation plant runs on natural gas with a maximized g524Riof 0.65. Total thermal efficiency is 0.91. The tri-generation system produces hot water at 140oC that is sufficient to run double effect absorption chillers at COPcHof about 1.25. The given distance between the supply and
49、demand is 0.6 km. From Equation 3: Checking the economic feasibility of the distance between the supply and demand points, where n is 0.60 from Equation 12-b for Trez= 280.15 K, the district distance is found to be acceptable. km 0.62010100011002.03.1maxg32g184g185g183g168g169g167g117g184g185g183g168g169g167g14g32nLDistrict Energy Scenario-2 In this scenario, waste heat 2 km away
