Heating power demand

That means that the average heating power demand becomes... 

Fig. 14 The components of heat power demand of drying air in a 1 MW boiler plant at different outdoor air temperatures. No heat recovery is used, the initial and final moisture contents of fuel are 60 % and 35 % and the temperature of drying air is 90 °C. The bars on the left represent the power needed to heat up the air and the bars on the right the power demand of the different stages strictly.

Fig. IS The rate of mass flow and heat power demand for heating of drying air as a function of the temperature of drying air.

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. 

Cogeneration systems will not match the varying power and heat demands at all times for most applications. Thus, an industrial cogeneration systems output frequently must be supplemented by the separate on-site generation of heat or the purchase of utility-supplied elec tric power. If the on-site electric power demand is relatively low, an alternative option is to match the cogeneration system to the heat load and contract for the sale of excess electricity to the local utihty grid. 

For a site with a fixed power demand throughout the year, the unfired plant illustrated in Fig. 9.2a is suitable for summer operation when the heat load is light. 

The simple back-pressure turbine provides maximum economy with the simplest installation. An ideal backpressure turbogenerator set relies on the process steam requirements to match the power demand. However, this ideal is seldom realized in practice. In most installations the power and heat demands will fluctuate widely, with a fall in electrical demand when steam flow, for instance, rises. 

If low-cost natural gas is available, a gas turbine can be used to generate power. In this case, the waste heat in the exhaust gas is used to produce steam in a heat recovery boiler (HR boiler). This approach also is used with some gas turbine plants (as in some high-speed navy vessels). Where an HR boiler is employed, if steam demand exceeds power demand, the boiler is fitted with auxiliary burners. 

Aero-derivative gas turbines are typically used for offshore applications where weight and efficiency are a premium, to drive compressors for natural gas pipelines, and stand-alone power generation applications for peak periods of high power demand. For stand-alone applications, gas turbine efficiency becomes a critical issue. However, if heat is to be recovered from the gas turbine exhaust, the efficiency becomes less important as the waste heat is utilized. 

When the temperature of 3He returning to the MC is higher than the MC temperature for more than about a factor 3, the dilution process does not work. Equation (6.13) clearly shows the importance of having efficient heat exchangers. For example, if h3 = 100 p,mol/s and the power demanded to the MC is 1 xW, we get ... 

The resulting electrochemical reaction produces a flow of electrons to provide the electrical power along with water and heat. The power is used by the orbiter s electrical system. The oxygen and hydrogen are consumed in the reaction in proportion to the orbiter s electrical power demand. 

In the chemical plant, the transformation of the feed streams (e.g., raw materials) into desired products and possible by-products takes place. In the heat recovery system, the hot and cold process streams of the chemical plant exchange heat so as to reduce the hot and cold utility requirements. In the utility plant, the required utilities (e.g., electricity and power to drive process units) are provided to the chemical plant while hot utilities e.g., steam at different pressure levels) are provided to the heat recovery system. Figure 7.1 shows the interactions among the three main components which are the hot and cold process streams for (i) and (ii), the electricity and power demands for (i) and (iii), and the hot utilities e.g., fuel, steam at different pressure levels, hot water) and cold utilities e.g., cooling water, refrigerants) for (ii) and (iii). 

The total mass of a future 50 kW fuel processor was estimated to be 55 kg (see Table 2.10), which corresponds to a total energy demand of 7 MJ for start-up heating. From this, a power demand for the air blower, which has to provide some 22 m3 min-1 of air to the system during start-up, was calculated to be about 1 kW. As this power is only required during the rapid start-up of the system (60 s), a normal battery could provide the power. An efficiency of 78% was calculated for the entire future fuel processor. 

Under all conditions, the capability to provide simultaneous heat and power provides substantial advantage for the microgrid system relative to the conventional system. The ability to effectively manage an H-APS to efficiently match both heat and power demand, including seasonal factors and unexpected events, is also key to the success of these systems. Installing technologies appropriately sized to deliver necessary heat and power as efficiently as possible will maximise benefits relative to conventional alternatives. 

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