Energy Conversion and Management 48 (2007) 809–818 www.elsevier.com/locate/enconman

A solid oxide fuel cell system for buildings Florian Zink a

c

a,*

, Yixin Lu b, Laura Schaefer

c

Department of Mechanical Engineering, University of Pittsburgh, 565 Benedum Engineering Hall, Pittsburgh, PA 15261, USA b NanoDynamics Inc., 901 Fuhrmann Boulevard, Buffalo, NY 14203, USA Department of Mechanical Engineering, University of Pittsburgh, 643 Benedum Engineering Hall, Pittsburgh, PA 15261, USA Received 23 February 2005; received in revised form 18 August 2006; accepted 10 September 2006 Available online 7 November 2006

Abstract This paper examines an integrated solid oxide fuel cell (SOFC) absorption heating and cooling system used for buildings. The integrated system can provide heating/cooling and/or hot water for buildings while consuming natural gas. The aim of this study is to give an overall description of the system. The possibility of such an integrated system is discussed and the configuration of the system is described. A system model is presented, and a specific case study of the system, which consists of a pre-commercial SOFC system and a commercial LiBr absorption system, is performed. In the case study, the detailed configuration of an integrated system is given, and the heat and mass balance and system performance are obtained through numerical calculation. Based on the case study, some considerations with respect to system component selection, system configuration and design are discussed. Additionally, the economic and environmental issues of this specific system are evaluated briefly. The results show that the combined system demonstrates great advantages in both technical and environmental aspects. With the present development trends in solid oxide fuel cells and the commercial status of absorption heating and cooling systems, it is very likely that such a combined system will become increasingly feasible within the following decade.  2006 Elsevier Ltd. All rights reserved. Keywords: Solid oxide fuel cell; Absorption heat pump; Heating; Cooling; Building

1. Introduction Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electrical energy. Fuel cells are a clean, quiet and efficient energy conversion technology and have been considered to be an advanced alternative to conventional combustion technologies for power generation. Fuel cells also may have high efficiencies even in small size units and are easy to site. Because of these features, fuel cells have recently been the focus of great interest as a distributed generation technology [1–4]. Solid oxide fuel cells (SOFCs) are one type of fuel cell and use ceramic materials as their electrodes and electrolyte. This allows SOFCs to work at high temperatures (up to 1300 K). Because of their high operating tempera*

Corresponding author. Tel.: +1 412 624 9766; fax: +1 412 624 4846. E-mail address: fl[email protected] (F. Zink).

0196-8904/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2006.09.010

ture, SOFCs are capable of incorporating internal fuel reformation, which allows multiple fuel options. Natural gas is one of the fuels that can be directly consumed by a SOFC system with an internal reformer [5,6]. SOFC systems also exhibit stable performance with a varying load [7–9]. The high temperature operation also makes SOFC systems less susceptible to fuel composition changes than low temperature systems. These features make SOFCs particularly suitable for distributed power generation and integration with other types of bottoming cycles such as gas turbine cycles and cogeneration [7,10–12]. A heat pump is an environmental energy technology that extracts heat from low temperature sources, upgrades it to a higher temperature and releases it for the required applications, such as space and water heating. Heat pumps can also be used in a reverse mode for cooling purposes, such as refrigeration and air conditioning. Heat pumps can be categorized into systems using vapor compression

810

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

Nomenclature [CH4] [CO] [CO2] cp COP f h Dhe Dhfuel Dhhc [H2] [H2O] K m_ n P P H2 O P CH4 PCO P CO2 Q T x y z Uf

molar flow rate of methane (mol s1) molar flow rate of carbon monoxide (mol s1) molar flow rate of carbon dioxide (mol s1) specific heat at constant pressure (J mol1 K1) coefficient of performance hydrogen flow rate at inlet of SOFC stack (mol s1) enthalpy (J mol1) combustion heat of hydrogen (J mol1) combustion heat of natural gas fed into combined system (J s1) combustion heat of consumed hydrogen in SOFC bundle (J s1) molar flow rate of hydrogen (mol s1) molar flow rate of steam (mol s1) reaction equilibrium constant mass flow rate (kg s1) total molar flow rate of gas mixture (mol s1) partial pressure of a gas in anode gas mixture partial pressure of steam in anode gas mixture partial pressure of methane in anode gas mixture partial pressure of carbon monoxide in anode gas mixture partial pressure of carbon dioxide in anode gas mixture heat (J) temperature (K) molar flow rate of the reacted methane in reforming reaction (mol s1) molar flow rate of reacted carbon monoxide in shifting reaction (mol s1) molar flow rate of reacted hydrogen in electrochemical reaction (mol s1) fuel utilization

cycles and absorption cycles. Absorption heating and cooling systems are thermally driven, which means that heat rather than mechanical energy is supplied to drive the cycle. Thermally driven absorption heating and cooling systems are attractive for using waste heat from other processes. They only require a small amount of electric power to drive the system, and they can use natural substances, which do not cause ozone depletion, as a working fluid. Fuel cell systems can be used as for combined heat and power (CHP) technology in buildings [13–16,42]. Proton exchange membrane fuel cells (PEMFC) and phosphoric acid fuel cells (PAFC) are widely used for CHP applications [15,16] but exhibit operation with electrical efficiencies only in the low 30% range. Furthermore, the lower operating temperature of a PEMFC requires the use of highly reformed fuels such as hydrogen and prohibits effective utilization of its waste heat for most absorption or

wAC wDC wH2 S

AC power generated from SOFC power generation unit (W) DC power generated from SOFC power generation unit (W) molar flow rate of hydrogen decomposed from H2S (mol s1)

Greek symbols g efficiency D difference Subscripts AB absorber AC alternating current CH4 methane CO condenser CO carbon monoxide CO2 carbon dioxide c cooling mode DC direct current e electrochemical reaction el electric EV evaporator GE generator H2 hydrogen H2O water vapor h heating mode r reforming f fuel s shifting stack fuel cell stack Superscripts i inlet o outlet

water heating technologies. We will show that a higher temperature SOFC system has greater advantages for the combined supply of heating and electricity over these technologies. As mentioned above, SOFC systems with internal reformation can be directly powered with natural gas, a fuel that already has an extensive supply infrastructure. It also emits flue gas at high temperatures, which allow it to be used as input for an absorption heating and cooling system. We propose a CHP design that combines a SOFC with an absorption heating and cooling system to produce power, heating/cooling and/or hot water for buildings while consuming natural gas. The high temperature exhaust and high efficiency (electrical and thermal) of our system render it superior to conventional low temperature CHP systems as described in Refs. [4,16,42]. For large residential or commercial buildings, such as office buildings, apartment

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

buildings, hospitals, swimming pools, supermarkets, etc., a combined system such as our proposed system will be very promising in the near future. 2. System configuration and description The basic system configuration of the combined system is shown in Fig. 1. The system consists of a desulfurizer, an internal reforming SOFC stack, a combustor, and an absorption heating and cooling system. After being desulfurized, natural gas is internally reformed, and the product, a hydrogen rich gas mixture, is fed into the SOFC anodes. Simultaneously, the air is pre-heated and fed into the SOFC cathodes. In the SOFC stack, the electrochemical reaction occurs at the interfaces of the electrolytes and anodes. Electricity is generated by electrons flowing through the anodes and an external circuit to the interconnects and cathodes. The reaction heat is internally used to support the reformation reaction. The depleted fuel and air streams are then fed into the combustor, where the residual fuel gases (hydrogen H2, carbon monoxide CO and methane CH4) are combusted with the excess oxygen from the depleted air stream. The hot flue gas flows through the high temperature generator, and the thermal energy is transferred to the refrigerant-absorbent mixture in the generator. This heating process generates refrigerant vapor and strong working solution. The strong working solution falls into an absorber. The refrigerant vapor flows into a condenser and is condensed into a subcooled or saturated liquid in the condenser and then flows into the evaporator. The liquid refrigerant leaves the evaporator and is absorbed by the strong working solution in the absorber. The processes in the condenser and absorber reject heat, which can be used as

811

heating sources. The process in the evaporator absorbs the heat from low temperature sources. Thus, the heat pump process can be used for heating as well as cooling purposes. The flue gas from the generator is also used to heat the air and fuel streams fed into the SOFC stack, or it can bypass the absorption system to a water heater to produce hot water. 3. System modeling To calculate the thermodynamic properties and chemical composition of the gases at the inlet and outlet of the system’s components, the energy and mass balances and the system performance, a detailed system model must be created. This model must interrelate the internal reformation, the electrochemical reaction and the component heat and mass balances. 3.1. Internal reformer model Internal reforming is an attractive option that offers significant cost reduction, higher efficiencies and faster load responses for a SOFC. In a SOFC system, anode gas recycling can be used to provide steam to the reforming process, which occurs at the anode side of the SOFC stack. The reaction mechanisms for internal reforming of natural gas are: Reforming reaction : CH4 þH2 O ! COþ3H2 ðx mol=sÞ ð1Þ Shifting reaction : COþH2 O ! CO2 þH2 ðy mol=sÞ ð2Þ Electrochemical reaction : H2 þ 0:5O ! H2 O ðz mol=sÞ

Because of the high operating temperature and catalytic effect of the anode materials, the reforming and shifting reactions are assumed to be in equilibrium during SOFC operation, and the equilibrium constants can be expressed as functions of the partial pressures of the gas components: Internal-Reforming Solid Oxide Fuel Cell System

Absorption Heat Pump System

Electricity Power Conditioner

Q CO

Generator

Condenser

Expansion Valve

Air

Internal-Reforming Solid Oxide Fuel Cell Stack

Q GE Solution Pump

Combustor Q EV

Evaporator

Absorber

ð3Þ

Q AB Flue gas

Desulferizer

Natural gas

Fig. 1. Schematic diagram of the basic configuration of a combined system.

Air

812

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

P 3H2 P CO P CH4 P H2 O P H2 P CO2 Ks ¼ P CO P H2 O Kr ¼

Reforming: Shifting:

ð4Þ ð5Þ

The equilibrium constants can also be expressed as functions of the mole fractions of the gas components:

i

Kr ¼

i

ðð½CO þ x  yÞ=ðni þ 2xÞÞðð½H2  þ 3x þ y  zÞ=ðni þ 2xÞÞ i

i

ðð½CH4   xÞ=ðni þ 2xÞÞðð½H2 O  x  y þ zÞ=ðni þ 2xÞÞ i

Ks ¼

P 2r

ð6Þ

i

ðð½CO2 þ yÞ=ðni þ 2xÞÞðð½H2  þ 3x þ y  zÞ=ðni þ 2xÞÞ i

ð7Þ

i

ðð½CO þ x  yÞ=ðni þ 2xÞÞðð½H2 O  x  y þ zÞ=ðni þ 2xÞÞ

where superscript ‘i’ refers to the inlet, and subscripts ‘r’ and ‘s’ refer to the reforming and shifting reactions, respectively. ni is the total inlet molar flow rate of the gas mixture and x, y and z are the reacted molar flow rates of CH4, CO and H2 in the reactions given in Eqs. (1)–(3), respectively. These molar flow rates satisfy the following relationship: z ¼ U f ð3x þ yÞ

ð8Þ

where Uf is the fuel utilization in the SOFC stack. When the temperature is known, the equilibrium can be calculated from: log K ¼ AT 4 þ BT 3 þ CT 2 þ DT þ E

ð9Þ

where constants A, B, C, D and E are listed in Table 1. The reformation reaction is an endothermic process, and the shifting reaction is exothermic. The reaction heats for the reformation and shifting processes can be calculated using the following equations: Qr ¼ xðhCO þ 3 hH2  hH2 O  hCH4 Þ

ð10Þ

Qs ¼ yðhCO2 þ hH2  hCO  hH2 O Þ

ð11Þ

The net heat needed for the shifting and reformation reactions is provided by the electrochemical reaction in SOFCs. 3.2. SOFC model The SOFC model is based on a pre-commercial tubular SOFC combined heat and power system (SOFC-CHP) developed by Siemens-Westinghouse. This study focuses Table 1 Coefficients for calculation of the equilibrium constants [17,18]

A B C D E

on the thermodynamic aspects of the SOFC system. Operational data for this system is provided by Siemens-Westinghouse and is given in Table 2 [19]. The DC efficiency of the SOFC bundle (gDC) is defined in Eq. (12), where wDC is the DC power produced by the SOFC power generation unit and Dhhc is the combustion heat of the consumed hydrogen in the SOFC bundle.

Reforming

Shifting

2.63121 · 1011 1.24065 · 107 2.25232 · 104 1.95028 · 101 6.61395 · 101

5.47301 · 1012 2.57479 · 108 4.63742 · 105 3.91500 · 102 1.32097 · 101

gDC ¼

wDC Dhhc

ð12Þ

The total heat generated in the SOFC stack Qstack can be calculated by Eq. (13) Qstack ¼

wDC ð1  gDC Þ gDC

ð13Þ

This generated heat includes the reversible electrochemical reaction heat and the heat due to the irreversibility of the process. It provides the heat for internal reforming and pre-heating of the incoming reactants. The flow rate of hydrogen fed into the SOFC stack can be calculated as i fstack ¼

wDC U f gDC Dhe

ð14Þ

where Dhe is the enthalpy of formation of the electrochemical reaction given in Eq. (3). The enthalpy changes of the reactants and products due to temperature variation are calculated by Z T1 Dh ¼ cp dT ð15Þ T2

Because of the high operating temperature of a SOFC, it can be assumed that all of the reactants display ideal gas behavior and can, thus, be used with standard cp(T) values [20]. The system’s electrical efficiency for the combined system is defined as follows: Table 2 The operating parameters of a 110 kW tubular SOFC DC power (wDC) AC power (wAC) DC efficiency (gDC) AC efficiency (gAC) Fuel utilization (Uf) Stoichiometric air Stack temperature

110 kW 95.5 kW 53% 46% 85% 4 1000 C

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

gsystem ¼

wAC Dhfuel

ð16Þ

ð19Þ

gHeating

ð20Þ

Assuming steady state operation for each heat exchanging component in the absorption system, we can apply a standard mass and energy balance: X X Mass balance : m_ i  m_ o ¼ 0 ð21Þ X _ þ Q_ ¼ 0 Energy balance : mh ð22Þ

A conventional heat driven dual pressure absorption system consists of an evaporator, a condenser, a generator and an absorber, as shown in Fig. 1. When the refrigerant leaves the evaporator, it is absorbed by a second working fluid. This absorption process rejects heat. The liquid mixture of absorbent and refrigerant is pumped to a higher pressure and into the generator. Direct thermal energy from a high temperature source is transferred to the refrigerant/absorbent mixture in the generator. This heating process generates refrigerant vapor, which flows into the condenser. The remaining liquid absorbent is then throttled back to the lower system pressure as it falls from the generator into the absorber. The coefficient of performance (COP) is commonly used for evaluation of the performance of heat pumps, including absorption cycles. The COP of an absorption system in the cooling and heating modes is determined by the following relationships: QEV QGE Q þ QCO COP H ¼ AB QGE

wCooling Dhfuel wHeating ¼ Dhfuel

gCooling ¼

3.3. Absorption heating and cooling system model

COP C ¼

813

4. A case study 4.1. System configuration The CHP system investigated in this case study produces electricity, heating/cooling and/or hot water. As shown in Fig. 2, it combines a SOFC system, a water-lithium bromide (LiBr) absorption heating and cooling system and an additional heat exchanger in an operating configuration similar to that described in Section 2. The fuel cell model is based on a pre-commercial 110 kW tubular SOFC developed by Siemens-Westinghouse. This system consists of a desulfurizer, an internal-reforming solid oxide fuel cell stack, a combustor and two recovery heat exchangers. The field performance parameters of this SOFC are given in Table 2, see above. The performance of the LiBr absorption system in this case study is based on a commercial gas fired absorption heating and cooling system by Dalian Sanyo Refrigeration

ð17Þ ð18Þ

For a combined system, the cooling and heating efficiencies are different from the COP for a single absorption system. They are defined as follows:

Electricity

Power Conditioner

Condenser Low-Temperature Generator

High-Temperature Generator

16

7 Internal-Reforming Solid Oxide Fuel Cell Stack 5 4

2

15

8

6 Combustor

Flue gas

7 HX1

11

10

9 Hot Water

13 Air 3

Desulferizer Water Heater

HX2

Natural gas 1 Cold Water

12

Evaporator

Absorber 15 14

Solution pump 16

Fig. 2. Schematic diagram of the configuration of a specific combined system for the case study.

18 17

814

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

Co., Ltd., in China (product model No. DG-11GM). It has been available for more than ten years. This LiBr absorption system is a double effect cycle that includes the following major components: a first stage generator, a second stage generator, a condenser, an evaporator, an absorber, two recovery heat exchangers, throttling valves and a solution pump. The nominal operating parameters of the DG11GM system are given in Table 3 [21]. To match the heat and mass balances between the SOFC system and LiBr absorption system, the operating parameters of the heat pump must be scaled down to utilize a smaller volume. For this study, it is assumed that the COP can remain unchanged. Additionally, the heat exchanger in the generator should be modified from a gas fired configuration to one utilizing the high temperature flue gas from the SOFC system. The performance parameters of the system used in the case study are given in Table 4. The assumed operating parameters of the auxiliary components are listed in Table 5. 4.2. System simulation In this study, the pressures in all of the SOFC system components are assumed to be 1 bar, and the flow resistances in the pipelines and components in the SOFC system are neglected. The typical natural gas composition in the United States is 94.4% methane, 3.1% ethane, and 1.1% nitrogen and other components [22]. In this study, the natural gas is assumed to be pure CH4. The recirculation of the depleted flue gas from the cell stack exit for internal reforming is set to 58%. This results in a steam-to-carbon Table 3 Nominal operating condition of the LiBr absorption system (DG-11 GM, Dalian Sanyo Refrigeration Co., Ltd.) Parameters

Values

Cooling capability (kW) Heating capability (kW) Electricity consumed by pumps (kW) Heat energy from fired gas (kW) Heating water inlet temperature (K) Heating water outlet temperature (K) Cooling water inlet temperature (K) Cooling water outlet temperature (K) Heating COP Cooling COP

351.24 295.24 2.65 348.33 329 334 285 279 0.84 1.01

Table 4 Scaled operation condition of the LiBr absorption system Parameters

Values

Cooling capability (kW) Heating capability (kW) Electricity consumed by pumps (kW) Heat from flue gas after combustor (kW) Heating water inlet temperature (K) Heating water outlet temperature (K) Cooling water inlet temperature (K) Cooling water outlet temperature (K)

115.9 96.4 1.68 114.8 329 333 285 280

Table 5 The assumed performance values of the related system components Parameters

Assumed values

Heat exchanger efficiency (%) Combustor isentropic efficiency (%) The minimum temperature difference (DT) between the fluids in all heat exchangers (K)

90 95 50

ratio (S/C) in the reforming process of 2.5 < S/C < 3. To calculate the heat and mass balance of this combined system, the system model described in Sections 3.1–3.3 was developed and implemented into code using FORTRAN. 4.3. Simulation results The results of the simulation, as denoted by the temperatures, flow rates and composition of the fluids at each node of the system, as shown in Fig. 2, are listed in Table 6. The full model development of the SOFC model has been discussed in greater detail in previous works by this paper’s authors [29–33]. The SOFC model has been validated using both the limited data provided by Siemens-Westinghouse and by comparison with other validated simulation results [23,34–38]. The LiBr absorption system has been validated using data directly provided by the manufacturer [21]. The calculated performance of the system for all three operating modes is shown in Table 7. We can see that when the combined system consumes 0.0044 kg/s of natural gas and 0.4653 kg/s of air, it produces a net electrical power of about 94 kW and can provide either 115.9 kW of cooling, 96 kW of heating or 103 kW of water heating capability. The system’s electrical efficiency is 43.3%. The space heating and cooling efficiencies are 43.7% and 52.6%, respectively. If we define total system efficiency as the sum of the electrical efficiency and the heating/cooling efficiency, we can see that the total system efficiency can reach 87% or more. These high system efficiencies are due to both the high level of utilization of the waste heat and to the COP (and therefore ambient energy usage) of the heat pump. 5. Discussion 5.1. Technical aspects Different types of buildings have different requirements for space heating, cooling, water heating and electricity. The combined system must be designed to fit different buildings. Given the energy consumption (Table 8) of a typical American office building, the combined system in the case study has the potential to supply the space heating, cooling, hot water and lighting electricity for an office building of around 9500 m2. An alternative configuration to the described thermally driven LiBr absorption heat pump could be a heat pump powered by electricity (i.e. a vapor compression cycle).

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

815

Table 6 The thermo-fluid properties at each state point for a SOFC power system Temperature (K)

Flow rate (kg/s)

Gas composition (mol%)

300 1073 300 1019 1173 1173 1348 1143 1143 1120 348

0.0044 0.0044 0.4577 0.4577 0.0147 0.4439 0.4653 0.4653 0.4653 0.4653 0.4653

CH4: CH4: N2: N2: H2O: N2: H2O: H2O: H2O: H2O: H2O:

100 100 79 79 59.8 83.4 2.8 2.8 2.8 2.8 2.8

Heating mode 14 329 16 333

5.54 5.54

H2O: H2O:

100 100

Cooling mode 17 285 18 280

5.54 5.54

H2O: H2O:

100 100

1 2 3 4 5 6 7 8 9 10 11

Table 7 Summary of calculated performance of the system in the case study

Natural gas (LHV) (kW) Electrical power (kW) Maximum heating (kW) Maximum cooling (kW) Maximum hot water (kW) Consumed pump work (kW) Net electric power (kW) System electrical efficiency (%) Heating efficiency (%) Cooling efficiency (%) Hot water efficiency (%) Total system efficiency (%)

Heating mode

Cooling mode

Hot water mode

220.4 95.5 96.4 0 0 1.68 93.8 43.3 43.7 – – 87

220.4 95.5 0 115.9 0 1.68 93.8 43.3 – 52.6 – 95.9

220.4 95.5 0 0 103 2.0 93.5 43.3 – – 46.7 90

O2: O2: H2: O2: O2: O2: O2: O2: O2:

21 21 9.8 16.6 15.9 15.9 15.9 15.9 15.9

CO:

11.0

CO2:

19.4

N2: N2: N2: N2: N2:

80.0 80.0 80.0 80.0 80.0

CO2: CO2: CO2: CO2: CO2:

1.3 1.3 1.3 1.3 1.3

membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC) and molten carbonate fuel cells (MCFC). PEMFCs and PAFCs, however, work at low temperatures such as less than 100 C and 200 C, respectively. Furthermore, both need an external fuel reformer in order to use fuel other than pure hydrogen and have a low tolerance for CO, which is a product of natural gas reforming [25]. MCFCs are a better option, since they work at high temperatures (900 K), and can incorporate internal fuel reformers. MCFCs are also good candidates for general stationary power generation. To evaluate the effect of using alternate fuel cells, the system interactions must be studied in detail. 5.2. Economic aspects

Table 8 Typical energy consumption intensities (W/m2) in office buildings in the USA (2003) [24] Space heating

Space cooling

Water heating

Lighting

87

3.32

3.18

10.26

When examined as an isolated system, a vapor compression cycle can be highly efficient. However, large conversion losses can occur in creating the work that drives the compressor. Thermally driven absorption cycles do not convert heat to work (electricity) and then back to heat, thus avoiding those energy losses. Nevertheless, mechanical heat pump systems have some advantages, and they are possible choices in some situations. For example, load matching is more complicated when the excess electricity produced cannot be sent to the grid, or the SOFC unit can not produce enough exhaust heat to meet all of the heating, cooling and hot water requirements. This is discussed further in Section 5.2 below. There are also alternatives to SOFCs that could be used in an integrated system, such as proton exchange

The thermodynamic analysis shows that the combination of a SOFC power generation system and a thermally driven absorption heating and cooling system results in a high overall efficiency. However, being economically viable is also very important for such a combined system to find wide practical applications. Based on the case study, the operating costs of such a combined system for several load cases were evaluated. The evaluation of operating cost (US $) considers energy costs in Western Pennsylvania in 2003. Natural gas and electricity prices were set to the retail prices of the commercial sector in that region: $329.47/1000 m3 and 8.38 ct/kW h, respectively [24]. Based on weather data for the Western Pennsylvania region [26], we can calculate the yearly energy costs and savings per day of operation. The economics of electrical load following versus space heating load following were compared. The results are discussed below. Figs. 3 and 4 show the typical load data for electricity and space conditioning for an office building in Pennsylvania [39].

816

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818 120.0

Electricity Heat

Energy [kW]

100.0

Table 9 Operating cost of discussed plant for a typical summer day (a) and winter day (b) Cost [$/day]

Electricity matched

Cooling matched

60.0

CHP base Additional heat Additional electricity Total

170 27 N/A 197

148 N/A 24 172

40.0

Separate production

272

20.0

(b) Winter CHP base Additional heat Additional electricity Total

173 54 N/A 227

Separate production

304

(a) Summer

80.0

0.0 0

3

6

9

12

15

18

21

24

Time [hr]

176 N/A 16 192

Fig. 3. Typical energy demand of an office building during a winter day.

Electricity Cooling

Energy [kW]

100.0 80.0

60.0 40.0

20.0 0.0 0

3

6

9

12

15

18

21

24

Time [hr]

Fig. 4. Typical energy demand of an office building during a summer day.

Using this data, the discussed fuel cell and heat pump plant’s performance is investigated for two different load cases in each season. First, the electricity demand is matched exactly, and we investigate how the heating demand is met. Second, the heating demand is matched exactly, and we investigate how the electricity demand is met. For the typical summer day, we perform the same investigation with electricity and cooling demand. In the case of under-produced heating or cooling, it is assumed that additional fuel is needed to supply the difference. Overproduced heating and cooling is considered waste. If electricity is under-produced, it is assumed that additional electricity has to be purchased from the grid. On the other hand, if we produce excess electricity, it can be supplied to the grid at half the purchase price. The data for the combined production of electricity and heating/cooling are then compared to separate productions. The results of these investigations are presented in Table 9(a) and (b). It can be seen that the combined production of heat and electricity or cooling and electricity decreases the energy

cost considerably. From this data, we can now calculate the overall savings and economic benefits. Considering the investment and maintenance costs as well as the operating cost savings, the years to payback for an added SOFC generator are dependent on the cost of the SOFC system (Fig. 5). At present, the initial cost of fuel cell based power systems is still high and is the biggest obstacle hindering large scale commercialization. In 1998, the cost per kWel of a 1.8 MWel SOFC unit was approximately $500-600/kWel, which includes cell stacks, a spent fuel recirculation loop, a pre-reformer, insulation, instrumentation and a stack container [27]. Considering peripheral system components, installation and maintenance costs, the total estimated cost per kW will be two to three times higher. The life span of a SOFC stack has been estimated to be between 4 and 8 years. The US Department of Energy’s Office of Fossil Energy is funding the Solid State Energy Conversion Alliance (SECA) SOFC programs. One major objective is to reduce

12

10

Years to payback

120.0

8

6

4

2 200 400

600

800 1000 1200 1400 1600 1800 2000 2200 Cost of SOFC unit (dollars/kW)

Fig. 5. The relation of the years to payback for an added SOFC system and the cost of the SOFC unit.

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

817

Table 10 Economical comparison between the proposed and conventional CHP systems

Investment cost ($/kWel) Maintenance cost ($/kW h) Life span (h) a b

Proposed system

ICE

Gas turbine

1000 0.0013 40,000a

1000–3000 0.0008-0.0013 15,000–40,000

1200 0.0006–0.0012b 40,000

Based on 8000 h/year [40]. Using a conversion rate of 1.25 Euro/ US$.

Table 11 Summary of emission reduction of combined system

6. Conclusion

Emissions (kg/day)

NOx

SOx

CO

CO2

95.5 kW (fossil fired plant in US)a 95.5 kW (US average) Single LiBr absorption systemb Combined system

58 – 2.6 0

39 – 0.005 0

39 – – 0

2346 1394 497 953

a b

The emissions data of a fossil fired plant provided by [28]. The emissions data of burning natural gas provided by [24].

the initial cost of SOFC systems to $400/kWel by 2010 [1]. If this target can be reached, the total estimated cost of a SOFC system will be less than $1000/kWel. At that cost and a SOFC life span greater than 5 years, this system will be a good alternative to current CHP technologies. Table 10 shows a comparison between the proposed CHP system and conventional CHP technology [40,41]. The data provided in Table 10 illustrate our claim that the proposed system compares well with other CHP technologies with respect to investment cost and maintenance cost. Assuming fuel cell technology will advance more in the short term future than both internal combustion engines and gas turbines, as they are both well established, it becomes clear that CHP systems, such as the proposed one, will likely become feasible in the near future. 5.3. Environmental aspects The burning of fossil fuels to meet our energy demand is resulting in the emission of vast amounts of air pollutants, which will have dramatic effects on our lives. However, through electrochemical reactions, SOFC power generation systems produce very low levels of air pollutants and greenhouse gases such as nitrous oxides (NOx), sulfur oxides (SOx) and CO [28]. LiBr absorption systems produce no ozone depleting substances. This combined system is very environmentally friendly. When comparing the emissions of a CHP system with a separate power generation system and a single LiBr absorption system with the same volumes, it can be seen that the system discussed in this work has great advantages over separate heat and electricity generation. The results of this comparison are given in Table 11. The comparison shows that the CHP system from our case study can reduce emissions of NOx, SOx, CO, and CO2 dramatically. The NOx, SOx and CO emissions are very small compared to the other emissions and can be neglected. CO2 emissions are reduced by about 55%.

An integrated system that combines a SOFC power generation system with an absorption heating and cooling system has been analyzed. The preliminary energy and mass balance analysis shows that such a system has the capacity to produce electric power, heating and/or cooling for buildings. The total system efficiencies in different modes are expected to reach 87% or more. Such a system demonstrates great advantages in both technical and environmental aspects. Since thermally driven absorption systems are already commercially mature products, the viability of a combined system depends on the commercialization of SOFC systems (which directly correlates to a decrease in the initial cost). With the present development trends in fuel cells, it is likely that initial costs for the SOFC will reach reasonable levels, rendering a CHP system, as proposed in our case study, commercially viable within the following decade. The economical and environmental analysis of the proposed system illustrates how it is superior to current power and heat supply technologies. References [1] Williams MC, Strakey JP, Singhal SC. U.S. distributed generation fuel cell program. J Power Sources 2004;131:79–85. [2] Bauen A, Hart D, Chase A. Fuel cells for distributed generation in developing countries—an analysis. Int J Hydrogen Energy 2003;28:695–701. [3] Dufour AU. Fuel cells – a new contributor to stationary power. J Power Sources 1998;71:19–25. [4] Cragg CT. The changing nature of the power generation market— does it create opportunities for fuel cells. J Power Sources 1996;61:1–6. [5] Meusinger J, Riensche E, Stimming U. Reforming of natural gas in solid oxide fuel cell systems. J Power Sources 1998;71:315–20. [6] Eguchi K, Kojo H, Takeguchi T, Kikuchi R, Sasaki K. Solid State Ionics 2002;152–153:411–6. [7] Costamagna P, Magistri L, Massardo AF. Design and part-load performance of a hybrid system based on a solid oxide fuel cell reactor and a micro gas turbine. J Power Sources 2001;96:352–68. [8] Kharton VV, Naumovich EN, Tikhonovich VN, Bashmakov IA, Boginsky LS, Kovalevsky AV. Testing tubular solid oxide fuel cells in nonsteady-state conditions. J Power Sources 2002;79:242–772. [9] Thorstensen B. A parametric study of fuel cell system efficiency under full and part load operation. J Power Sources 2001;92:9–16. [10] Palsson J, Selimovic A, Sjunnesson L. Combined solid oxide fuel cell and gas turbine systems for efficient power and heat generation. J Power Sources 2000;86:442–8. [11] Tanaka K, Wen C, Yamada K. Design and evaluation of combined cycle system with solid oxide fuel cell and gas turbine. Fuel 2000;79:1493–507.

818

F. Zink et al. / Energy Conversion and Management 48 (2007) 809–818

[12] Winkler W, Lorenz H. The design of stationary and mobile solid oxide fuel cell—gas turbine systems. J Power Sources 2002;105:222–7. [13] Seara F, Vazquez M. Study and control of the optimal generation temperature in NH3–H2O absorption refrigeration systems. Appl Thermal Eng 2001;21:343–57. [14] Alanne K, Saari A. Sustainable small-scale CHP technologies for buildings: the basis for multi-perspective decision-making. Renew Sustain Energy Rev 2004;8:401–31. [15] Gigliucci G, Petruzzi L, Cerelli E, Garzisi A, Mendola AL. Demonstration of a residential CHP system based on PEM fuel cells. J Power Sources 2004;131:62–8. [16] Kazim A. Economical and environmental assessments of proton exchange membrane fuel cells in public buildings. Energy Convers Manage 2001;42:763–72. [17] Massardo AF, Lubelli F. Internal reforming solid oxide fuel cellgas turbine combined cycles (IRSOFC-GT). Part A. Cell model and cycle thermodynamic analysis. J Eng Gas Turbines Power 2000;122:27–35. [18] Bossel UG. Final report on SOFC data facts and figures. Swiss Federal Office of Energy, Berne, CH; 1992. [19] George RA. Status of tubular SOFC field unit demonstrations. J Power Sources 2000;86:134–9. [20] Perry RH, Green DW, Maloney JO. Perry’s chemical engineers’ handbook. 7th ed. New York: McGraw-Hill; 1997. [21] Dalian Sanyo Refrigeration Co., Ltd. Available from http://www. dl-sanyo.com/. [22] Spath PL, Mann MK. Life cycle assessment of a natural gas combined-cycle power generation system. NREL/TP-570-27715, 2000. p. 10. [23] Campanari S. Thermodynamic model and parametric analysis of a tubular SOFC module. J Power Sources 2001;92:26–34. [24] Energy Information Administration, DOE. Available from www.eia.doe.gov. [25] National Energy Technology Laboratory. Fuel cell handbook. 5th ed. Morgantown, WV. 2000. [26] National Oceanic and Atmospheric Administration Availble from http://www.cdc.noaa.gov. [27] George RA, Bessette NF. Reducing the manufacturing cost of tubular solid oxide fuel cell technology. J Power Sources 2000;71: 131–7.

[28] Stambouli AB, Traversa E. Solid oxide fuel cells (SOFCs): a review of an environmentally clean and efficient source of energy. Renew Sustain Energy Rev 2002;6:433–55. [29] Lu Y, Schaefer L, Li PW. Numerical simulation of heat transfer and fluid flow of a flat-tube type high power density solid oxide fuel cell. ASME J Fuel Cell Sci Technol 2005;2:65–9. [30] Lu Y, Schaefer L, Li PW. Numerical study of a flat-tube high power density solid oxide fuel cell: Part I. Heat/mass transfer and fluid flow. J Power Sources 2005;140:331–9. [31] Li PW, Schaefer L, Chyu MK. A numerical model coupling the heat and gas species’ multiple transport processes in a tubular SOFC. ASME J Heat Transfer 2004;126:219–29. [32] Lu Y, Schaefer L. A SOFC system fed with hydrogen sulfide and natural gas. J Power Sources 2004;135:184–91. [33] Li PW, Schaefer L, Chyu MK. Multiple transport processes in solid oxide fuel cells. In: Sunden B, Fahgri M, editors. Transport phenomena in fuel cells. Southampton: Wit Press; 2005. [34] Lu Y. Numerical simulation of a flat-tube high power density solid oxide fuel cell. Ph.D. Dissertation 2004, University of Pittsburgh. [35] Li PW, Schaefer L, Chyu MK. Interdigitated heat/mass transfer and chemical/electrochemical reactions in a planar type solid oxide fuel cell. In: ASME summer heat transfer conference 2003, Las Vegas, July, HT2003-47436. [36] Singhal SC. Advances in solid oxide fuel cell technology. Solid State Ionics 2000;135:305–13. [37] George RA. Status of tubular SOFC field unit demonstrations. J Power Sources 2000;86:134–9. [38] Williams MC, Strakey JP, Singhal SC. U.S. distributed generation fuel cell program. J Power Sources 2004;131:79–85. [39] National Renewable Energy Laboratory. Available from www.nrel. gov/docs/fy05osti/34931.pdf. [40] Beausoleil-Morrison I, Knight I, Urgursal I. Residential cogeneration systems: a review of the current technologies. Annex 42 of the International Energy Agency, Energy Conservation in Buildings and Community Systems Programme, 2005. [41] Pilavachi PA. Mini- and micro-gas turbines for combined heat and power. Appl Thermal Eng 2002;22:2003–14. [42] Lokurlu A, Grube T, Ho¨hlein B, Stolten D. Fuel cells for mobile and stationary applications—cost analysis for combined heat and power stations on the basis of fuel cells. J Hydrogen Energy 2003;28:703–11.

A solid oxide fuel cell system for buildings - Semantic Scholar

Fuel cells are a clean, quiet and efficient energy conversion technology and have ... that extracts heat from low temperature sources, upgrades .... 2. System configuration and description. The basic system configuration of the combined system.

223KB Sizes 0 Downloads 67 Views

Recommend Documents

discrete numerical simulations of solid oxide fuel cell ...
18 Comparison of nonstoichiometry data sets . ... 19 Fits of a random defect model to LSM20 nonstoichiometry data . . . . . . . 42 ...... Community Activism.

A solid oxide fuel cell system fed with hydrogen sulfide ...
C [5]. To circumvent some of these issues, an H2S decomposi- tion reactor (DR) integrated with an SOFC system ... metal-membrane reactor can drive the decomposition of H2S ..... pacity to recover the heat from SOFC power generation sys-.

Cell Proliferation - Semantic Scholar
Jun 28, 2005 - 318-342-1958; Fax: 318-342-1606; E-mail: [email protected] ... levels of phospho-MEK1 (active) and phospho-ERK1/2 (active) in these cells.

pdf-138\advanced-fuel-cell-system-thermal-management-for-nasa ...
... the apps below to open or edit this item. pdf-138\advanced-fuel-cell-system-thermal-management-for-nasa-exploration-missions-by-kenneth-a-burke.pdf.

pdf-138\advanced-fuel-cell-system-thermal-management-for-nasa ...
... the apps below to open or edit this item. pdf-138\advanced-fuel-cell-system-thermal-management-for-nasa-exploration-missions-by-kenneth-a-burke.pdf.

A 3D Shape Measurement System - Semantic Scholar
With the technical advancement of manufacturing and industrial design, the ..... Matlab, http://www.vision.caltech.edu/bouguetj/calib_do c/. [3] O. Hall-Holt and S.

A Scalable Messaging System for Accelerating ... - Semantic Scholar
using these HEC platforms in the paper. Keywords-associative messaging system, publish/subscribe, in- situ/in-transit analytics, data staging. I. INTRODUCTION.

Cell Phone Radiofrequency Radiation Exposure ... - Semantic Scholar
Feb 23, 2011 - signal exposure on brain glucose metabolism. JAMA. 2011;305(8):808-813. 2. Anderson V, Rowley J. Measurements of skin surface temperature during mo- bile phone use. Bioelectromagnetics. 2007;28(2):159-162. 3. Straume A, Oftedal G, John