Rejected Heat Utilization

The cell and stacks that compose the power section have been discussed extensively in the previous sections of this handbook. Section 9.1 addresses system processes such as fuel processors, rejected heat utilization, the power conditioner, and equipment performance guidelines. System optimization issues are addressed in Section 9.2. System design examples for present day and future applications are presented in Sections 9.3 and 9.4 respectively. Section 9.5 discusses research and development areas that are required for the future system designs to be developed. Section 9.5 presents some advanced fuel cell network designs, and Section 9.6 introduces hybrid systems that combine fuel cells with other generating technologies in integrated systems. 

By contrast. Fig. 13.46 shows an endothermic reactor integrated below the pinch. The reactor imports Qreact from part of the process that needs to reject heat. Thus integration of the reactor serves to reduce the cold utility consumption by Qreact- There is an overall reduction in hot utility because, without integration, the process and reactor would require (Qumin + Qreact) from the utility. 

Thus, a cogeneration system is designed from one of two perspectives it may Be sized to meet the process heat and other steam needs of a plant or community of industrial and institutional users, so that the electric power is treated as a by-produc t which must be either used on site or sold or it may be sized to meet electric power demand, and the rejected heat used to supply needs at or near the site. The latter approach is the likely one if a utility owns the system the former if a chemical plant is the owner. 

Rejection of waste heat is a feature of most chemical processes. Once the opportunities for recovery of heat to other process streams and to the utilities system (e.g. steam generation) have been exhausted, then waste heat must be rejected to the environment. The most direct way to reject heat to the environment above ambient temperature is by... 

Although a fuel cell produces electricity, a fuel cell power system requires the integration of many components beyond the fuel cell stack itself, for the fuel cell will produce only dc power and utilize only processed fuel. Various system components are incorporated into a power system to allow operation with conventional fuels, to tie into the ac power grid, and often, to utilize rejected heat to achieve high efficiency. In a rudimentary form, fuel cell power systems consist of a fuel processor, fuel cell power section, power conditioner, and potentially a cogeneration or bottoming cycle to utilize the rejected heat. A simple schematic of these basic systems and their interconnections is presented in Figure 9-1. 

Without co-generation the utility consumption would be Qpp. Therefore, the work W is get for free The transformation of high-pressure steam in work has a theoretical efficiency of 100% when the rejected heat is used inside a process. Co-generation has proved to be cost effective in processes involving high exothermal reactions, but also as a method for valorisation of process waste. 

Heat transfer area. The flooding of the silo with molten salt increases the effective surface area of heat transfer from the reactor vessel to the silo wall. If the silo is full of molten salt, the entire silo wall, not a small section of the wall, rejects heat to the environment. The placement of the reactor core at the very bottom of the reactor vessel allows foil utilization of the complete silo area. Because molten salt heat fluid is used for heat transfer, heat rejection rates can be further increased by (1) increasing silo depth or (2) designing the top of the silo with its shorter pathway for heat rejection to the environment. The effective heat transfer area is thus doubled. 

Below the pinch, the system rejects heat and so is a net heat source. When a heat recovery system design does not have cross-pinch heat transfer, that is, from above to below the pinch, the design achieves the minimum hot and cold utility requirement under a given ATmin-... 

The production of desalinated water or heat for district heating utilizes reject heat from the Brayton cycle and does not degrade the S-CO2 cycle efficiency for electricity production. The absence of a low pressure turbine and condenser, as in a Rankine steam cycle, means that the CO2 exiting the low temperature recuperator has an elevated pressure and temperature that facilitates coupling to a bottoming cycle desalination plant or a district heating heat exchanger in which the CO2 is cooled to 31.25°C immediately above the CO2 critical temperature in preparation for compression to maximum pressure. 

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