Configuration Optimization with Operation Strategy of Solar-assisted Building Cooling Heating and Power System to Minimize Energy Consumption

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Peer-review under responsibility of the organizing committee of CUE 2015 doi: 10.1016/j.egypro.2016.06.063

Energy Procedia 88 ( 2016 ) 742 – 747

ScienceDirect

CUE2015-Applied Energy Symposium and Summit 2015: Low carbon cities and urban

energy systems

Configuration optimization with operation strategy of

solar-assisted building cooling heating and power system to

minimize energy consumption

Jiangjiang Wang

a,b

*, Tianzhi Mao

a

, Chao Dou

a

SchoolofEnergy,PowerandMechanicalEngineering,NorthChinaElectricPowerUniversity,Baoding,Hebei,071003,China

b

InstituteofEngineeringThermophysics,ChineseAcademyofSciences,Beijing,100190,China

Abstract

This work designs a novel hybrid building cooling heating and power (BCHP) system incorporating with solar energy and natural gas. A basic natural gas BCHP system containing power generation unit, heat recovery system, hybrid cooling system and storage tank is integrated with solar photovoltaic (PV) and/or thermal collector. Optimization methodology with genetic algorithm (GA) is applied to optimize the configuration of the solar-assisted BCHP system for minimizing primary energy consumption. BCHP schemes are optimized in following electrical load (FEL) and following thermal load (FTL) operation strategies respectively. The result indicates that BCHP system in the FEL mode consumes less energy consumption than in the FTL mode to meet building demands. But the FTL mode would be superior to FEL mode at taking the surplus products from the hybrid BCHP system into consideration.

Keywords: Building cooling heating and power (BCHP) system; solar energy; integration; optimization

1. Introduction

Energy demands in building mainly consist of electricity for lighting and appliance, space heating and cooling, and domestic hot water. Building cooling heating and power (BCHP) system is one of energy-efficient options to satisfy building demands, but also improve energy unitization efficiency. Solar energy integrated into gas-fuelled BCHP system is an efficient method to produce continuous power and simultaneously reduce carbon dioxide (CO2) emissions. Solar energy is usually integrated to BCHP

system as electrical energy or thermal energy. The electricity can be directly produced by solar

* Corresponding author. Tel.: +86-312-7522443; fax: +86.-312-7522440

E-mail address: [email protected]

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photovoltaic (PV) arrays. However, the integration forms of solar thermal energy are various, such as heating work fluid [1, 2], solar thermochemistry [3], and driving absorption chiller [4]. BCHP system itself is a comprehensive design, especially equipment configuration [5], capacity optimization [6, 7], and operation strategy [8]. Furthermore, the integration and complementation of solar energy and fossil energy makes energy flow and system structure complicated. Currently, the researches on solar-assisted system mainly focus on the incorporating form of solar energy [9, 10], and energy or exergy performance improvement [11], etc. The literature survey shows that very few studies focus on the optimization of solar-assisted CCHP system. The originality of this work lies in proposing an optimization methodology with genetic algorithm (GA) for optimizing the configuration of solar-assisted hybrid BCHP system to minimize its energy consumption.

2. Solar-assisted BCHP system

Electricity flow Fuel flow Thermal flow Solar energy flow

Egrid Qrc _Building Boiler Qc Qh Fb Qb Qrh Qr Heat recovery system Power generation unit Fpgu Epgu Eec Storage Tank Qs,in Qs,out Absorption chiller Electrical chiller Qec Qac Qex Fgas Utility grid Solar PV Solar heat collector EPV Qc Heating exchanger Eb FPV FPV Fsolar Fcoal Qex-gas

Fig. 1. Solar-assisted hybrid BCHP system

The energy flow of the solar-assisted hybrid BCHP system is shown in Fig. 1, which consists of electricity generation subsystem, thermal producing subsystem, waste heat utilization subsystem for cooling and heating. Electricity is generated from natural gas power generation unit (PGU) and solar PV arrays, and the electricity grid also supplement. Thermal producing subsystem includes heat recovery system, solar heat collectors, thermal storage tank and gas boiler. The first heat source is from the heat recovery system that is recovered the waste heat from the high temperature exhausted gas of PGU, the solar heat collectors are the second heat source, and the boiler is being a backup heat source. The thermal storage tank is used as an adjustable heat source to store or release heat in case of surplus heat. The cooling subsystem consists of absorption chiller and electric chiller. The absorption chiller uses waste heat to provide cooling while the electric chiller coverts electricity to cooling.

The operation strategy of this hybrid system mainly includes the following rules:

(1) The electricity generated by solar PV arrays and the heat collected by solar collectors are both firstly used to meet the building demand.

(2) In case the supply from solar PV arrays and heat collectors is insufficient, the PGU will run. The PGU operation strategies can be classified to following electric load (FEL) and following heat load (FTL).

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(3) The utility grid is used to supplement electricity in case of insufficient electricity from PGU and solar PV arrays, but also it is sent the surplus electricity from hybrid BCHP system back.

(4) The recovered heat from PGU is firstly consumed, and then the heat stored in heat tank is used when the heat is not enough.

(5) The absorption chiller always runs and provides cooling for base cooling load while the electric chiller is used to supplement cooling at the peak load.

3. Optimization methodology

A superstructure is configured in the hybrid BCHP system as shown in Fig. 1. The energy demands are supplied by several alternative ways. The inappropriate configuration will result in energy waste or bad economic state. Analyzed the capacity configurations in this BCHP system, the four main equipments including PGU, solar PV arrays and heat collector, heat storage and absorption chiller are vital to influence the capacity configuration and operation status of other equipments. Consequently, the capacity configuration of these decision variables can be expressed to:

1 [ ,max, ,max, ,max, ,max, ,max]

T

pgu pv sc storage ac

X N N N N N (1)

where Npgu_,max, Npv,max, Nsc,max,Nstorage,max and Nac,max are the installation capacities of PGU, solar PV

arrays, solar heat collectors, heat storage tank and absorption chiller, respectively.

The capacities of PV arrays and solar heat collector are related closely because the available solar area in a building is generally limited. To lessen the number of decision variables, the solar PV ratio is defined to:

pv total

A A

T (2)

where T is the solar PV ratio, A is PV arrays’ area and pv Atotal are the harvest solar area in building. Thus, the solar PV ratio can replace the two parameters including Npv,max and Nsc,max.

Additionally, BCHP systems, especially those operating stand-alone, are subject to variable demands of electrical power, heating and cooling loads. A key problem between design and operation is that the operation characteristics may be far away from the design condition. For example, the energy-efficiency of PGU usually varies with the load factor and will dramatically drop at the low load. If the building load is too low, BCHP system may result in energy waste and bad performance. So, the on–off coefficient of PGU , Į, is defined to control the PGU’s operation [12]. When the load factor is less than the optimized value, the PGU will stop and the electricity is supplemented by the grid.

Through the above analysis, the main optimization variables are integrated to:

,max ,max ,max

[ , , , , ]T

pgu storage ac

X N N N T D (3)

The energy use in the hybrid BCHP system includes solar energy, natural gas for PGU and coal for electricity from grid. To minimize the fossil energy consumption, the following optimization objective is constructed: min ( ) ( ), s.t. ( ) 0, 1, 2, , ( ) 0, 1, 2, , n gas coal X i i F F X F X X R h X i m g X i m m q d (4)

where F is the objective function, hi is equal constraint whereas gi is unequal constraint to ensure certain

process conditions. The equal constraints are mainly energy flow relationships. The unequal constraints include equipment characteristics and parameter limitations. To solve this optimization problem, GA is employed to search the optimal values. The optimization procedure is shown in Fig. 2.

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Building energy demands Solar radiation Technical parameters Initial conditions BCHP optimization configuration Problem No Satisfy objectives? Yes Decision variables Constraints Hybrid BCHP model: Energy consumption calculation Coding Decoding Next population Solution method Initialization Reproduction Crossover

Mutation _operationGenetic GA parameters BCHP Optimization Model BCHP configuration Objective function

Optimization result of solar-assisted BCHP system

Fig. 2. Optimization procedure

4. Case study 4.1. Initial conditions

The initial conditions are summarized in Table 1. The efficiency varying with load factor (f) is considered to represent the variable load operation [12-15].

4.2. Results and analysis

To solve this optimization problem, GA is employed to search the optimal values. The optimization results are listed in Table 2. It can be observed that the capacity of PGU in FTL mode is much larger than in FEL mode because the heat demand is larger than the electrical demand. Consequently, the PGU on-off coefficient in FTL mode is also larger to avoid the low load and bad energy efficiency. The solar PV ratio in FTL mode is also larger than in FEL mode, and thus, the solar heat collectors in FEL are larger. Furthermore, the surplus heat in FEL mode is produced. Consequently, the heat storage capacity in FEL mode is larger than in FTL mode. To hybrid cooling configuration, the cooling capacity driven by heat in FTL is more than in FEL mode because the recovered heat for PGU in FTL mode is larger. Totally, the energy consumption in FEL mode is less than in FTL mode.

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Table 1 Initialization conditions

Parameters Initialization

Building peak loads Electricity: 446 kW, Cooling: 1804 kW, Heating: 757 kW Solar peak radiation Scattered radiation 571 W m-2_{, Direct radiation: 964 W m}-2

Technical parameters PGU

generation efficiency 39% at full load,

5 4 3 2 , 5 + 4 3 + 2 1 + 0 e pgu f f f f f K D D D D D D 5=2.8725, 4= 9.0468, 3=11.1760, 2= 6.9889, D D D D D1=2.3782,D0= 0.000002 [13] Electric chiller COP _{5.05 at full load,} 3 2

3 2 1 0

ec

COP b f b f b fb

3 8.7389, 2 15.8250, 1 12.5080, 0 0.3618

b b b b [13]

Absorption chiller COP _{0.8 at full load,} 3 2

3 2 1 0

ec

COP c f c f c fc c3 1.1080,c2 2.4190,c1 1.6830,c0 0.4250

Solar PV efficiency _{14.44% at full load,} 2

, 1 2

d e PV d f d f

K d1 0.0237,d2 0.1681,d3 0.1078 [14]

Boiler _{93% at full load,}

, 1 0

e b g f g

K ,g1 0.00428,g0 0.922716 [15]

Solar heat collector 45.0% Heat recovery efficiency 80.0% Coal power plant

generation efficiency

35.0% Grid transmission efficiency 92.0% Heat storage loss 10.0%

Then analyzed their energy compositions and surplus products, the performances are listed in Table 3. It can be seen that the natural gas consumption is less and the consumed coal is more in FEL mode than in FTL mode because its PGU capacity is less and the supplemented electricity from grid is larger. Although BCHP system in FTL mode consume more energy than in FEL mode, its surplus electricity is much larger than in FEL mode. Concluded the surplus products to the benefits of hybrid BCHP system, the energy efficiency in FTL mode is larger than in FEL mode. The FTL mode is superior to FEL mode, but a large amount of surplus electricity must be sent back to grid.

Table 2 Optimization results

Parameters Npgu,max

, kW ,max storage N , kW ,max ac N , kW T Į Energy, GWh a -1 FEL mode 252 5185 840 0.46 0.07 3.444 FTL mode 691 751 1204 0.63 0.47 3.744 Table 3 Energy consumptions and surplus products

Parameters Natural gas _{, GWh} Solar energy _{, GWh} _{, GWh}Coal Surplus electricity, _MWh Surplus heat, _MWh _{product, MWh}Total surplus FEL mode 0.22 0.01 3.08 4.32 0.55 4.87 FTL mode 1.42 0.01 2.32 590.73 0 590.73

5. Conclusion

Genetic algorithm optimization methodology was employed to optimize the configuration of the solar-assisted BCHP system. The case study demonstrated its feasibility for optimizing configuration of hybrid BCHP system. To minimize the energy consumption, the PGU, heat storage, absorption chiller and solar PV arrays and heat collectors should be configuration optimally. The prime mover’s capacity in following thermal load mode is larger than in following electrical load mode due to the larger heat demand. Consequently the prime mover’s on-off coefficient will increase. But the hybrid BCHP system in following electrical load mode consumes less energy consumption than in following thermal load mode.

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If the surplus products from BCHP system can be utilized fully, the following thermal load mode is superior to following electrical load mode.

Acknowledgements

This research has been supported by National Natural Science Foundation of China (51406054).

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Biography

Jiangjiang Wang is an associate professor of School of Energy, Power and Mechanical Engineering at North China Electric Power University, China. His research interests include distributed energy system, combined cooling, heating and power system, and control technology for building environment.