The operation of a direct methanol fuel cell with an internal combustion engine in a hybrid system is investigated in terms of fuel efficiency. The following work shows a potential for fuel saving because the engine's waste heat is utilized in preconditioning of methanol for the fuel cell and in postconditioning of the cell's anode exhaust for the engine. The low activity of methanol oxidation catalysts and methanol crossover are the main drawbacks of direct methanol fuel cells. H3PO4-doped polybenzimidazole membranes have lower methanol crossover, and allow a higher operational temperature and methanol concentration compared to Nafion membranes. The operation of the cell at higher temperature with polybenzimidazole membranes improves catalyst activity and mass transfer increasing cell efficiency. But the fuel feed to this type of membrane must be in vapor phase. Methanol solution can be evaporated by the engine coolant. Unutilized methanol in the anode exhaust is converted to H2 rich product gas in a reactor before feeding into the engine. The endothermic reaction enthalpy for this conversion is recovered from engine's exhaust gas. The system efficiency increases with the cell's fuel utilization, as long as the cell's efficiency is higher than the engine's efficiency. In order to increase the system efficiency with load, the current density of the fuel cell should not be increased beyond the point where the cell and engine efficiency meet. Beyond that, the product gas should be substituted with liquid methanol to meet the rest of the load because the engine charge's energy density can be increased with liquid methanol injection into the engine. If the engine charge is comprised of fuel cell exhaust only and the engine's indicated efficiency is 20%, the efficiency of the hybrid system will be 25.5% at a cell voltage of 0.4 V and a cell fuel utilization of 40%. This corresponds to a fuel saving of 28% compared to the internal combustion engine. The hybrid system efficiency will increase to 28.5% at this operating point, if the fuel cell's anode exhaust is further decomposed in a reactor prior to combustion in the engine. The addition of the reactor to the hybrid system corresponds to a fuel saving of 43% compared to the engine and a fuel saving of 12% compared to the hybrid system without the reactor.

Fuel cells operated with hydrogen are more efficient than internal combustion engines, because the combustion in the internal combustion engine is less reversible than the electro-oxidation of hydrogen in the fuel cell. Hydrogen can be produced out of hydrocarbons, such as natural gas, or renewable resources at stationary facilities, but fuel cells operated with pressurized hydrogen stored on board require advanced hydrogen infrastructure for commercialization. An alternative to on-board storage of hydrogen is on-board processing of a liquid hydrocarbon to hydrogen via steam reforming. However, on-board reforming of hydrogen is less energy-efficient than centralized production of hydrogen. Moreover, on-board reforming, purification, and subsequent oxidation of the reformate in a fuel cell is not more efficient than a hybrid electric vehicle technology assisted internal combustion engine. We propose a fuel cell-heat engine hybrid system, which consists of a membrane reformer, a fuel cell, and a reciprocating internal combustion engine, and estimate the efficiency of the proposed hybrid system. Steam reforming of a hydrocarbon requires additional heat input, which can be recovered from the waste heat of an internal combustion engine. On the other hand, the retentate of the membrane reformer can be used in the internal combustion engine to further increase the system efficiency. Methanol is proposed as the fuel for the membrane reformer because the temperature level required is low enough to recover waste heat of reciprocating internal combustion engines for steam reforming of methanol. The hybrid system proposed is more flexible than a fuel cell with an on-board reformer, because additional fuel can be directly combusted in the internal combustion engine at cold start or rapid load increase. Because fuel cell efficiency decreases with load and internal combustion engine efficiency increases with load, the overall system efficiency is less load-dependent compared to the efficiencies of each of these technologies. The power of an automobile engine is considered as a benchmark for the system proposed.

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...

This article investigates theoretically the products from adiabatic combustion of synthesis gas reformed out of aqueous methanol with a variable gasoline−methanol mixture assuming chemical equilibrium. Previous research has generally focused on the combustion of a given fixed fuel composition, whereas in this article the composition of the fuel is varied by three different parameters: the amount of water in the aqueous methanol used as the synthesis gas feedstock, the amount of methanol in the gasoline−methanol mixture, and the ratio of synthesis gas energy to total fuel energy. The effects of these parameters on the adiabatic flame temperature and combustion product distribution are determined. The method used in this article allows the above parameters and the air/fuel ratio parameter to vary in equation sets derived using Lagrange undetermined multipliers in order to determine the combustion temperature and products with a computer solution. Stoichiometric combustion of synthesis gas reformed out of an equimolar water−methanol mixture causes a decrease in the NO ratio by 20% compared with methanol and 40% compared with gasoline under equilibrium conditions, whereas the adiabatic flame temperature decreases by only 50 and 100 K, respectively. Stoichiometric combustion of synthesis gas reformed out of neat methanol causes a higher adiabatic flame temperature and higher NO x emissions than stoichiometric combustion of gasoline or methanol. However, neat-methanolbased synthesis gas has a higher power density and extends the lean flammability limit of the engine more efficiently than aqueous-methanol-based synthesis gas because the reforming enthalpy of neat methanol is higher than that of aqueous methanol. Hydrogen-rich synthesis gas fuels should be combusted with lean air/fuel ratios at part load to increase the thermal efficiency, combustion stability, and control emissions simultaneously. If the road load increases, the engine charge's energy density and motor octane number will be increased by substitution of synthesis gas with liquid fuel while decreasing the air/fuel ratio simultaneously.

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