Cogeneration efficiency

Figure 23.44 The definition of cogeneration efficiency and site power-to-heat ratio. (From Varbanov P, Perry S, Makwana Y, Zhu XX and Smith, 2004, Trans IChemE, 82A 784, reproduced by permission of the Institution of Chemical Engineers.)...

However, a more useful measure of the utility system performance is the cogeneration efficiency. Of the fuel fired in the utility system, some of this energy produces power, some provides useful process heat and some is lost. The cogeneration efficiency recognizes the amount of fuel consumed to produce both power and useful process heat, and can be defined as1516 ... 

While this basic definition of cogeneration efficiency seems straightforward, complications are created by the process steam generated from waste heat recovery that can be used for power generation or process heating and that does not require any fuel to be fired in the utility system. The heat supply can be defined as the sum of the heat from fuel (both in the utility boilers and fired heaters) and steam generation from the waste heat recovery (see Figure 23.44)17 ... 

The cogeneration efficiency is therefore more correctly defined as ... 

This section covers system issues such as HHV, LHV, and cogeneration efficiency calculations, heat rate calculations, and cogeneration steam duty calculations. 

Thermal heat (MWt) Electricity consumed (MWe) Electricity cogeneration efficiency IS plant net efficiency (HHV)... 

System Configuration steam header pressures, steam turbine configration, maximal cogeneration efficiency. 

No matter how much the steam system could be optimized, one relevant and important question is What is the maximal efficiency that the steam system can achieve The answer will be provided by the concept of cogeneration efficiency that is discussed next. 

Clearly, cogeneration efficiency is a function of R ratio and the steam system configuration. Although the steam system is operated with the objective of achieving minimum cost, the maximum efficiency provides a measure of the best efficiency which the system can reach for a given configuration. 

The R ratio and cogeneration efficiency based on the current operation in Figure 18.3 can be calculated as... 

You may ask What is the steam system configuration that can achieve the best cogeneration efficiency for a given process steam and power demands In the previous discussions, we know that the maximum cogeneration features maximal back-pressure power generation and minimal condensing. To achieve this, let us define the ideal R ratio, which is expressed as... 

However, what is the improved cogeneration efficiency from these changes Let us do the numbers. 

Thus, the gap in cogeneration efficiency between the maximal cogeneration efficiency (Figure 19.6) and the current operation (Figure 19.1) is 18% (67%-49%). How much is it worth in terms of MMBtu/h Does it tell something else ... 

Until now, we can safely derive the conclusion It is not always tme that maximal efficient operation is the most economical operation. It is tme only when words, the necessary and sufficient condition can be stated as When the PEE is higher than on-site power generation efficiency, the maximal efficient operating mode is also the minimal cost operating mode. Under this condition, the problem of optimizing the steam system is converted to a simplified problem to maximize the cogeneration efficiency. 

That could explain the reason why the minimal cost operation in Figure 19.3 favors maximal condensing flow, where the fuel price is 4/MMBtu and power price is 90/MWh. This is because the above condition does not hold for this operation as the PEE for power import is 15.2%, which is worse than that of the condensing path. Thus in this case, the condensing path beats the power import in cost but at the expense of cogeneration efficiency. 

They also evaluated the competitiveness compared with gas engines, of which Wartsila is a major supplier. The lifetime of the stacks was assumed to be 40,000 and 50,000 h in 2015 and 2020, respectively. The cost of the system was predicted to be between 680 and 1,080 /kW in 2020 when the cost of a gas engine would be 660 /kW. The efficiency of the SOFC was, however, calculated to be 55% and the net electric and total cogeneration efficiency 90%. For the gas engine, the corresponding numbers were 37% and 81%, respectively, so that the SOFC plant would indeed be the most economical. 

This section covers performance issues such as higher heating value (HHV), lower heating value (LHV), cogeneration efficiency, heat rate, and cogeneration steam duty calculations. 

Before calculating the cogeneration efficiency, first determine the heat duty associated with steam production. This requires knowledge of the steam and feed water enthalpies, which can be found in the ASME Steam Tables (11) as indicated below ... 

Two main advantages of the phosphoric add fuel cell include a cogeneration efficiency of nearly 85 % and its ability to use impure hydrogen as fuel. PAFCs can tolerate a carbon monoxide concentration of about 1.5 % which increases the number of fuel types that can be used. Disadvantages of PAFCs include their use of platinum as a catalyst (like most other fuel cells) and their large size and weight. PAFCs also generate low current and power comparable to other types of fuel cells [13]. 

The energy utilization factor (EUF) and the cogeneration efficiency are given by... 

---