Integrating Fuel Cell Power Systems Into Building Physical Plants

J. Carson KCI Technologies, Inc.

Hunt Valley, MD 21030

This paper discusses the integration of fuel cell power plants and absorption chillers to cogenerate chilled water or hot water/steam for all weather air conditioning as one possible approach to building system applications. Absorption chillers utilize thermal energy in an absorption based cycle to chill water. It is feasible to use waste heat from fuel cells to provide hydronic heating and cooling. Performance regimes will vary as a function of the supply and quality of waste heat. Respective performance characteristics of fuel cells, absorption chillers and air conditioning systems will define relationships between thermal and electrical load capacities for the combined systems. Specifically, this paper develops thermodynamic relationships between bulk electrical power and cooling/heating capacities for combined fuel cell and absorption chiller system in building applications.

The absorption cycle

Absorption chillers use water vapor at high vacuum as the refrigerant. For discussion, point 1 in the cycle would be the low pressure refrigerant as saturated liquid at approximately 40F. The refrigerant enters the evaporator and removes heat from the working fluid - chilled water - to provide the latent heat for transition to saturated vapor. At point 2, the refrigerant enters the absorber, where it comes into contact with the absorbent - a salt solution with a high affinity for water. The solution is then compressed through a pump and passed to the generator where waste heat effectively boils the refrigerant off to a superheated state at point 3. Once the superheated vapor leaves the generator, it passes through the condenser - point 4 - and the cycle again resembles more conventional refrigeration cycles._

Table 1 Refrigerant Properties

2 1.354 190 Vapor 1,143.5 BTU/LB

3 1.354 112 Sat. liquid -80 BTU/LB

4_0.122_40_Throttled_-80 BTU/LB

Table 1 represents state properties of the refrigerant of some typical absorption machines currently available. These machines normally employ an ammonia or lithium bromide solution as the absorbent. Because of the toxicity of ammonia, absorption chillers usually use the lithium bromide solution. During compression and heating, the solution is nominally 54 percent by weight lithium bromide. Once the refrigerant has been "boiled off" in the generator, the remaining solution is 59 percent by weight lithium bromide. When considered from the absorbent side of the cycle, it is essentially a regenerating solvent cycle, where the "strong liquor" absorbs refrigerant in the absorber to become a "weak liquor" and then is regenerated in the generator to become again the "strong liquor." Table 2 contains the performance characteristics for the model used in the analysis.

Table 2

Model Chiller Operational Parameters

Generator vacuum Evaporator vacuum Mass fractions

"Strong" liquor "Weak" liquor Generator heat Steam Hot water

340F, 28 PSIA 260F supply (THWS) 210F return (THWR)

Tables 1 and 2 define the model absorption chiller used in the analysis. Mass and heat balances show that the specific effective heat input QR is 15.73 MBTOH/ton cooling, for a coefficient of performance of 0.763. Mass flow rates for the "weak" and "strong" liquors are 141.S LBn/HR/ton and 129.6 LB0/HR/ton, respectively. These flows result in 12 LBn/HR/ton refrigerant flow.

System Analysis

This analysis treats the cases of molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs), because they represent technologies nearer to the market. In both MCFC and SOFC models, anode gas is oxidized in a catalytic burner. The oxidized anode gas and cathode gas are then used to pre-heat incoming fuel and oxidant streams. After the preheat heat exchangers, the streams are combined and pass on to the waste heat recovery system. The waste heat stream is then exhausted, although in the MCFC variant, make-up C02 for the cathode is recovered prior to exhaust.

The following are mass balances for the catalytic oxidation of anode gas in MCFCs and SOFCs, respectively:

(1+4UF) C02 + 4 (1-UF) H2 + (4UP+m-2) HjO + 2 (1+Y) (1-UP) 02

+ 7.52 (1+Y) (1-UF)N2 -> (1+4UF) C02 + (4+m-2)H20 + 2Y(1-Up) 02 + 7.52 (1+Y) (1-UF)N2 (1)

4 (1-Up)H2 + (4UF+m-2)H20 + C02 + 2 (1+Y) (1-UP) 02

+ 7.52 (1+Y) (1-Up)N2 -> (4+m-2)H20 + 2Y(1-Up)02 + 7.52 (1+Y) (1-Up)N2 + C02 (2)

In each equation, Up is the fuel utilization factor, m is the steam to carbon ration and Y is excess air. The mass balances account for internal reforming, high temperature shift and power reactions. Table 3 contains performance parameters for model MCFC and SOFC power plants. Applying these parameters to equations (1) and (2) defines the compositions of the respective anode gas streams._

Table 3

Operational Parameters for Model Fuel Cells MCFC SOFC

T 1660R 2290R

U0XI 0.60 0.60

Following oxidation, anode gas temperature is:

0 0

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