Subsystem, capital costs

Another notable element of the cost increases is the inclusion in the Shaw flow sheets of subsystems for hydrogen product purification and for feed water purification. For S-I and to a lesser extent for HyS, concern with the carry-over of sulphur to the product necessitated a significantly costly purification subsystem on the product end of the cycle. For HTSE, the issue is uncertainty in the purity of input steam and effects on the electrolysis cells, and so a large feedwater purification subsystem impacts the overall capital cost. In the earlier evaluations these subsystems were not considered significant enough to include. 

Consider an example of Evans, et al., which consists of three subsystems in an overall system, as shown in Figure 3a (14). Figure 3b shows the system with the subsystems separated and ready for economic optimization with the X s representing unit available energy values at the points indicated. To do this rigorously, the capital costs and input available energy rate of each subsystem must be expressed as functions of the output available energy rate and the decision variables X particular to that subsystem. 

Let us consider only two special cases among those considered by Evans, et al. (14). For the case in which the subsystem capital costs are insensitive to the output available energy rate and each subsystem cost depends only on the thermodynamic efficiency mi, then the result of the mathematical optimization is that Xj = cj,... 

Consider the situation where the subsystem capital costs increase linearly with output available energy,... 

Table 2 shows the information available in the literature for capital costs of the reformer (excluding the PSA), hydrogen compressor, storage tanks and dispenser island scaled to a hydrogen capacity of 10,000 std m per day (900 kg of H2 per day or 1249 kW of H2 on a EHV basis). Scaling factors were applied to determine the costs of the units as the manufactured quantities increase. The relationships between hydrogen capacity and subsystem capital cost are also shown in Table 2. 

Table 2. Capital cost for refueling station subsystems as a function of the manufactured quantities. The delivered hydrogen capacity is 67 kg/day.

Figure 2. Contribution of Subsystems to Capital Cost for 115 kg/day HFAs. (The "Miscellaneous" category includes on-site installation, freight, taxes insurance, and initial spares. The "Reformer System" category includes the hydrogen production and gas cleanup subsystems.)...

GE EER and GE s Global Research Center (GRC) will analyze the design for reliability (DFR) of the fuel processor. The components that require a design review to reduce capital costs have been identified. DFR templates that have been developed by GRC to record all failures will be used. This data will be analyzed and all identified reliability issues will be addressed during the design of the next system. The DFR plan also includes component testing that will help identify failures of subsystems and components. Information from DFR analysis will be combined with the economic analysis to determine the economic benefits of the advanced fuel processor system. 

A comparative cost analysis was performed for the AHTR by scaling individual subsystem costs for either the GT-MHR or the S-PRISM. The result is that the AHTR overnight capital cost (without contingency) is estimated to be approximately 820 /kW(e) (2002 dollars), which is 50-55% of the S-PRISM and GT-MHR costs for similar total output. This is a consequence of economy of scale. The AHTR electrical output is approximately four times that of these other reactors but with a similar plant size and complexity. Relative to light-water reactors, the AHTR should be more economical because of the higher power conversion efficiency, low-pressure containment, and absence of active safety systems. 

The number of cells in a stack is determined by consideration and optimization of the power conditioning subsystem (to minimize power conditioning losses). For the 25 MW SOFC/GT system example (with 20 MW from the SOFC), the optimal maximum stack voltage is 400 V, which translates into the optimal number of cells per stack of 400-500. Given the cell size and the number of cells per stack, the optimal stack building block for the 25 MW plant is estimated to have a nominal power rating of about 320 kW. This system thus needs 64 stack building blocks these stacks, however, can be divided into modules to lower capital costs (multiple... 

The efficiency of the total fuel cell system can be calculated by accounting for the generation efficiency of the fuel cell stack, the efficiency of power conversion devices, and the accessory load of the related subsystems. In order to obtain the best overall fuel cell power system efficiency and better economics, a system optimization is required. The optimization involves minimizing the cost of electricity or heat and electric products as in a cogeneration system all the component processes of the system should be integrated into an efficient plant with low capital cost. Often, these objectives are conflicting, so compromises, or design decisions, have to be made. In addition, project-specific objectives, such as desired fuel, environmental emission levels, potential uses of rejected heat, desired output levels, volume (volume/kW)... 

It is noted that all the candidate compressors mentioned earlier are costly in capital, energy, and operations and maintenance (O M). Advances have centered on the optimization of subsystems for cost reduction and improvement of the energy efficiency of gaseous hydrogen compression. Centrifugal compression and electrochemical hydrogen... 

It should be mentioned that other methods of design optimization, employing the Second Law for costing, have been used. For example, without explicitly determining the cost of available energy at each juncture of a system, in 1949 Benedict (see 19) employed the Second Law for optimal design. He determined the "work penalties" associated with the irreversibilities in an air separation plant. That is, the additional input of shaft power to the compressors required as a consequence of irreversibilities was determined from the entropy production in each subsystem. Associated with additional shaft power requirements are the costs of the power itself and the increased capital for larger compressors. 

Each subsystem could then be optimally selected on the basis of the tradeoff between increased capital expenditure for reducing subsystem irreversibilities on the one hand, and increased power and compressor costs on the other hand. 

The sensor subsystem contributes an insignificant amount to the PFD of ESD2. That probably represents an opportunity to reduce the quantity of equipment and also reduce the lifecycle cost of manual proof testing. Comparing the failure rates of the switches that indicate closure of the valve to the failure rate of the radio system, it is clear that one of the two switches could easily be eliminated. This will reduce the false trip rate as well as lower capital and lifecycle cost. One could go further, however, by looking at the safety contribution of each of the sensor types. The PFD contribution of the ZS sensor subsystem (Gate Gil) and the PT sensor subsystem (Gate G12) is shown in Table 13-4. 

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