Waste heat rejection

Robertson, R. C. 1980. Waste heat rejection from geothermal power plants. In Kestin, J. (ed) Sourcebook on the Production of Electricity from Geothermal Energy. US Department of Energy, Washington DC, 997 p. 

Computer Simulation of Atmospheric Effects of Waste Heat Rejected From Conceptual Large Power Parks Bhumralkar, C. M. 

Heat engines operate in a thermodynamic cycle, which is simply a series of processes that repeat over time and use a working fluid, which is most commonly either a gas or a vapor. All heat engine cycles consist of a pressure increase process, a heat addition process where heat is added from the heat source, an expansion process where work is extracted, and a waste heat rejection process. In an attempt to improve the efficiency (the amount of work produced for a given heat input), the cycle can become greatly more complicated and include a number of intermediate processes. However, a heat engine must always include the basic elements of a heat supply and heat rejection. 

It is not a widely known fact that water withdrawals for the purpose of waste heat rejection from electricity-producing power plants can be equal to those for agricultural use (irrigation), see Fig. 11. The same may be true for hydrogen production plants. Power plant cooling water demands can be enormous [21]. For example, the Pacific Gas and Electric Diablo Canyon Station in the USA (two 1164 MW(e) pressurized water reactors) pumps 2 billion gallons (7.75 billion litres) of seawater per day for heat rejection and discharges the water at 20°F ( 11°C) hotter than the ambient sea temperature. 

Classification of fuel cells by temperature is becoming more blurred, however, since a current SOFC research focus is lower temperature (<600°C) operation to improve start-up time, cost and durability, while a focus of PEFC research has been to increase operation temperature to > 120°C to improve waste heat rejection and water management. The ideal temperature seems to be around 150-200°C which is where the PAFC typically operates. However, the PAFC has its own historical limitations which have hampered enthusiasm for its continued development. 

Atkins, Harold E., Pacific Northwest National Laboratory, PNL-47163-1-1, NaK 78) Annular Linear Induction Pump (ALIP) Design Study for Proposed Brayton Cycle Waste Heat Rejection System, Prepared for NASA GRC under contract JCN47163, June 2004 12-23 Idel chik, I. E., Handbook of Hydraulic Resistance," AEC-TR-6630, 1960 (Russian), 1966 (English)... 

An important by-product of most energy technologies is heat. Few energy conversion processes are carried out without heat being rejected at some point in the process stream. Historically, it has been more convenient as weU as less cosdy to reject waste heat to the environment rather than to attempt significant recovery. The low temperatures of waste heat in relation to process requirements often make reuse impractical and disposal the only attractive alternative (see Process energy conservation). 

Hea.t Pumps. The use of heat pumps adds a compressor to boost the temperature level of rejected heat. It can be very effective in small plants having few opportunities for heat interchange. However, in large faciHties a closer look usually shows an alternative for use of waste heat. The fuel/steam focus of energy use has led to appHcation of heat pumps in appHcations where a broader examination might suggest a simpler system of heat recovery. 

The first step is energy conservation, which is the subject of Chapter 34. Recovery of rejected or wasted heat requires a careful analysis of the heat flow within the systems under survey Points to examine are ... 

Fhosphoric acid does not have all the properties of an ideal fuel cell electrolyte. Because it is chemically stable, relatively nonvolatile at temperatures above 200 C, and rejects carbon dioxide, it is useful in electric utility fuel cell power plants that use fuel cell waste heat to raise steam for reforming natural gas and liquid fuels. Although phosphoric acid is the only common acid combining the above properties, it does exhibit a deleterious effect on air electrode kinetics when compared with other electrolytes ( ) including such materials as sulfuric and perchloric acids, whose chemical instability at T > 120 C render them unsuitable for utility fuel cell use. In the second part of this paper, we will review progress towards the development of new acid electrolytes for fuel cells. 

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... 

FIG. 24-65 Mechanical refrigeration TIC systems utilizing chilled water for cooling turbine inlet air, and cooling towers to reject the waste heat into the environment, account for the majority of refrigeration TIC systems sold. 

The results show that, at temperatures below 60 °C and an air feed stoichiometry below three, the cathode exhaust is fully saturated (nearly fully saturated at 60 °C) with water vapor and the exhaust remains saturated after passing through a condenser at a lower temperature. In order to maintain water balance, all of the liquid water and part of the water vapor in the cathode exhaust have to be recovered and returned to the anode side before the cathode exhaust is released to the atmosphere. Because of the low efficiency of a condenser operated with a small temperature gradient between the stack and the environment, a DMFC stack for portable power applications is preferably operated at a low air feed stoichiometry in order to maximize the efficiency of the balance of plant and thus the energy conversion efficiency for the complete DMFC power system. Thermal balance under given operating conditions was calculated here based on the demonstrated stack performance, mass balance and the amount of waste heat to be rejected. 

The amount of waste heat to be rejected from an operating stack constrains in a significant way the optimal DMFC stack operating conditions and therefore should be considered when designing the complete power system in order to minimize the size and weight and maximize the power and total energy conversion efficiency. 

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