Combined Heat and Power

Combined Heat and Power systems allow the production of useful heat and electricity from the same source to be utilised.  Thus, increasing the overall efficiency through recovery of the heat rejected from the inefficient energy conversion of producing electricity.  This is no new concept, with the use of CHP systems well known if under promoted.  A traditional CHP system can be seen below in figure 1 below.

  Figure 1 – Traditional CHP system layout

The main components of a typical system are:

·        the engine or prime mover that drives the generator.

·        the generator that produces electricity.

·        the heat recovery system that recovers the waste heat from both the engine water cooling jacket and exhaust gases.

·        the exhaust system to take away the products of combustion.

·        the control panel to monitor the operation.

Typical characteristics of the main CHP systems in operation are summarised below.

Table 1 – Summary of some traditional CHP systems characteristics.

This arrangement of recovering rejected heat can be carried over to fuel cell operation.  Mainly through the development of two main types as mentioned earlier, Solid Oxide (SOFC) and Phosphoric Acid (PAFC).  For the purpose of CHP applications these two types will be the focus, although in some circumstances the Proton Exchange Membrane (PEM) cell and Molten Carbonate cell could be considered. 

The reasoning for focusing on the two types is the commercially availability and interest in development of the PAFC and the high operating temperature and interest in the SOFC.  Moreover, they allow for a comparison between a relatively low operating temperature and a high operating temperature cell. 

Solid Oxide Fuel Cell

The development of the SOFC system has been increased recently but there is no commercially available plant.  Much of the data in the case of the SOFC is from published studies and testing.  In some instances assumptions have been made and highlighted.  As mentioned earlier the fuel cells are named after the electrolyte where this case is a ceramic that is solid.  This arrangement was derived for the reduction in corrosion properties within the cell through the use of the ceramic yttria stabilised zirconia.  The SOFC examined by ETSU was by Westinghouse.

Operational Characteristics

The operating characteristics can be seen in table 2, below.

Table 2 – Summarised characteristics of SOFC.

The SOFC cost data can be summarised below in table ? from an ETSU study below.  However, the cost of the stack and other components where estimated to be £100,000 and the maintenance costs are comparable to that of a working PAFC.

Table 3 – Associated costs of SOFC.

Cell design and layout of components

Phosphoric Acid Fuel Cell

ONSI corporation is the sole provider of fuel cell power plants for CHP commercially, the PC25.  This cell has gone through extensive trials through a US government grants in cooperation with the Ministry of Defence.  The published findings and details of their use can be seen at www.dodfuelcell.com. 

Operational Characteristics

The operating characteristics can be seen in table4, below.

Table 4 – Summarised characteristics of PAFC.

1/ The PC25 consumes 580 m³ natural gas per hour. The gas line is sized for up to 914 m³ per hour to accommodate increased fuel consumption as the fuel cell stack efficiency degrades over time.  The PAFC power plant requires a natural gas delivery pressure of 101 to 355 mm of water column.

Table 5 – Associated costs of PAFC.

Cell design and layout of components

Comparison of PAFC and SOFC cells

In order to compare both fuel cell arrangements, a theoretical scenario where the application of both the outputs from the cells can be utilised and compared in terms of costs and benefits.  Therefore, by using a medium size office building that the needs is known to us we can consider their performance.  The assumed building parameters are listed below in table?.

Methodology to Comparison

1.      Characteristics of Fuel Cell as described in the operational characteristics of the cells

2.      Determine the outputs of both cells

3.      Determine the building requirements and assess the displaced thermal and electrical loads

4.      Determine the energy savings and subsequently the payback period

5.      Assess the viability of both cells with changes in parameters, sensitivity analysis.

Calculation

Table 6 – Required building data  

2/ Minimum demand is no less than the cells rating to reduce the need for selling excess into the grid system.

Table 7 – Comparison of Fuel Cells performance  

Envirnomental Considerations

The Carbon dioxide emissions saved by the two fuel cells are the PAFC saving 1,794,328 kg/kg and for the SOFC 1,242,877 kg/kg.  This is taking the value for a large scale generation of electricity.

Sensitivity Analysis

The sensitivity analysis was carried out with changes to the following parameters and are graph below:

• 1. Change in floor area
• 2. Change in cost of fuels
• 3. Changes in the heat recovery performance

1.      Change in Floor Area

2. Change to costs of fuels

 3. Change in the performance of the heat recovery

Conclusions

·        The PAFC is less capital intensive as they have been on the market for many years whereas the SOFC is still under development

·        The SOFC is overall more efficient than the PAFC due to its higher electrical efficiency.

·        Not all the rejected heat from the PAFC is available at a temperature that is useful for effective use.

·        Neither Fuel Cell will have an acceptable payback period below the fuel consumption of a building below 4000m².

·        A difference of thirty percent in cost of electricity will have a change in 2 years on the payback period for the SOFC and 3 years on the PAFC.

·        A difference of thirty percent in cost of natural gas will have a change in ½ years on the payback period for the SOFC and 1 years on the PAFC.

·        The PAFC is susceptible to change in performance of the heat recovery due to the influence of the electrical performance.

Comparison of PAFC and traditional gas CHP system

Operational Characteristics

Table 8 - Operating characteristics of gas turbine CHP system

Costs

Table 9 - Costs associated with gas turbine CHP system

Calculations

Table 10 – Comparison of Fuel Cells performance

Conclusions

·        The capital costs of the technology play a considerable role in determining the economic viability of a propose system.

·        The maintenance costs of the gas turbine are higher due to more moving parts and being a dirtier technology

·        The fuel cell would be more profitable in the long term, greater than 15 years, due to the considerable annual net savings.  Around 30% in this time.