Combined Cooling Heating and Power (CCHP) Generation in a Fuel Cell-Heat Pump Hybrid System

Abstract

The utilization of fuel cells in stationary decentralized power systems necessitates reforming of existing fuels to hydrogen, if those fuel cells may not be directly operated with commercial fuels like natural gas. Though it is possible to operate high temperature fuel cells like solid oxide and molten carbonate fuel cells directly with natural gas, those have longer start up times, compared to low temperature fuel cells like polymer electrolyte fuel cells (1). Therefore it is more convenient to utilize low temperature fuel cells for small scale residential power production. On the other hand, electric power production out of renewable or fossil resources in power plants and its subsequent distribution is more efficient than reforming of such fuels to hydrogen and its subsequent electro-oxidation in a PEM Fuel Cell to produce only distributed electric power at the site of demand.But if a fuel cell is utilized for combined cooling heat and power, the overall efficiency in terms of coefficient of performance for heating or cooling will be better, compared to direct firing of a fuel for heating purposes. In cogeneration systems, the efficiency of energy conversion increases to over 80% as compared to an average of 30 -35% for conventional fossil fuel fired electricity generation systems. (2) Whereas the cooling load of a low temperature fuel cell can hardly be utilized for cooling but for heating purposes, the exhaust heat of the reformer may be utilized for heating and cooling via an absorption heat pump.Cogeneration applications in buildings have to satisfy either both the electrical and thermal demands, or satisfy the thermal demand and part of the electrical demand, or satisfy the electrical demand and part of the thermal demand. Depending on the magnitude of the electrical and thermal loads, whether they match or not, and the operating strategy, the cogeneration system may have to be run at part-load conditions, the surplus energy (electricity or heat) may have to be stored or sold, and deficiencies may have to be made up by purchasing electricity (or heat) from other sources such as the electrical grid (or a boiler plant) (3). Such a cogeneration system should follow the heating or cooling demand instead of the power demand in order to be operated independent from the grid, without using a storage system and at its maximum possible coefficient of performance. The electric power production results according to the system's thermal efficiency, the ratio of the electric power produced to the fuel energy reformed rated by its lower heating value. If following of cooling or heating demand would cause a surplus of local electric power production, this could be sold back to the electric network. But if the electric power market is not liberal enough for a decentralized power production network or selling is economically unfeasible, a compression heat pump can consume the surplus of electric power for additional heating or cooling to decrease fuel consumption of the reformer, while meeting the same heating/cooling dem...

CCHP System Performance Based on Economic Analysis, Energy Conservation, and Emission Analysis

“…Before electrooxidation at the fuel cell's anode, only permeated hydrogen needs to be cooled. Both the absorption and compression heat pumps use the fuel cell's waste heat and electricity for heating and cooling applications [29]. Moreover, high-temperature fuel cells combined with an absorption chiller offer the potential to meet the criterion of virtually zero pollutant emissions [30].…”

“…where, LPM is the lifetime of the PM [29]. Another discounted cash flow method is the net present value (NPV) for CCHP systems.…”