Techno-economic Analysis and Optimization
The investigated BIO-CCHP technology delivers electricity at comparably high efficiencies (>40%) and heat. Besides, the recovered heat can also be used to produce additional electricity i.e. to improve electric efficiency of the system in e.g. an organic Rankine cycle (ORC) and for cold production with an absorption machine which is gaining increasing importance due to global warming.
Data for the operation of a gasifier-gas engine CHP system for a downdraft gasifier (small system, with a performance of about 1 MW) and a gasifier-gas engine-ORC system for a fluidized bed gasifier (large system, with a performance of about 10 MW) was collected as reference cases. In a next step, the gas engines were replaced by SOFCs and an additional cold generator (absorption or an alternative compression chiller) was considered for the generation of cold in the proposed CCHP system. Assumptions and selections made for the heat recovery process were based on literature reviews. A range for CAPEX and OPEX for SOFC systems estimating possible future cost scenarios (low cost: many commercial plants, optimized production costs; high costs: as today, few plants built per year) were included in the analysis. All cases where modelled and simulated in Aspen Plus, whereby the model inputs (i.e. temperature, gas composition, volume flows and internal heat/power demands) were based on data generated within the project. Life cycle cost assessments (LCC) were performed for each case based on energy and material outputs from Aspen Plus and economic correlations from literature.
Table 1 shows values for performance and costs of the small system initially estimated for the project proposal, compared with the values according to the outcomes of the project (small and large system). The electric output of the system is on par with expectations, whilst the thermal and cold power are lower for the small system. The costs for the system are within the ballpark of the proposal estimation, at least for the long term. For the short to medium term the costs are a bit higher, which also make the specific costs per kW generally higher. Also, for the large system there is a clearly lower specific cost. This is however not mainly due to scale, but rather due to the higher efficiency in this system than the small.
Table 1. Comparison of project expectations according to the proposal and the results of assessments made in the project. Produced heat and cold taken into account for €/kW calculation by dividing it by 5 and 2.5 as weight factors, respectively. Future cost scenarios considered (low to high SOFC costs). FDA...fixed bed downdraft air; FBS...fluidized bed steam
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FROM PROPOSAL |
RESULTS FROM THE PROJECT |
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Parameter |
Unit |
Bio-CHP (FDA gasifier + engine) |
Bio-CCHP (FDA gasifier + SOFC + chiller) |
Bio-CCHP Small System (FDA gasifier + SOFC + chiller) |
Bio-CCHP Large System (FBS gasifier + SOFC + chiller) |
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Technical |
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Fuel input |
MW |
1 |
1 |
1.02 |
12.55 |
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Electrical power |
MW |
0.275 |
0.42 |
0.41 – 0.44 |
5.1 – 5.8 |
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Electrical efficiency |
% |
27.5 |
42.0 |
40.2 – 43.1 |
40.6 – 46.2 |
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Thermal power |
MW |
0.36 |
0.235 |
0.12 – 0.15 |
1.3 – 3.1 |
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Cold power |
MW |
- |
0.17 |
0.09 – 0.11 |
1.0 – 2.3 |
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Economical |
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Costs for electric- and cooling units |
k€ |
330 |
975 |
915 – 2409 |
8545 – 24141 |
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CAPEX |
k€ |
1850 |
2680 |
2415 – 3909 |
24565 – 40141 |
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€/kW(Pel + Q̇heat/5 + Q̇cold/2.5) |
5330 |
5010 |
4788 – 8796 |
3563 – 6877 |
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OPEX |
k€/a |
220 |
242 |
305 – 376 |
3360 – 4085 |
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€/kW (Pel + Q̇heat/5 + Q̇cold/2.5) |
635 |
452 |
604 – 843 |
487 – 700 |
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Concluding, three main questions had to be answered in this project regarding power generation, cold generation and energy storage:
- Can the SOFC - system be competitive with a Gas - Engine - system?
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Answer: YES. The large system is already today competitive, and the small system will be so in the medium to long term. |
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But: There are large uncertainties in the assessment that need to be addressed by anyone wanting to analyze the potential of the system further.
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- Does absorption cooling make sense in the system, or should power production be maximized and cooling be done with a compression cooler?
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Answer: Absorption cooling makes sense. There is a lot of waste heat that needs to be utilized for a high total system efficiency and absorption cooling increases both efficiency and economic results. |
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But: It could also be shown that the waste heat available from the system is at a high temperature (out from the SOFC). Thus, there needs to be more high-quality use of the waste heat in order to reach a high exergetic efficiency in the system. Concluding, the best system configuration in this study would be when Rankine cycles for increased power production are combined with absorption cooling. There is still a need/potential for improving and optimizing this use of waste heat in the system. |
- Can energy storage add value to the concept?
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Answer: Yes. Storage is an important aspect to consider matching the supply from the BIO-CCHP plant with the demand variability and thus have a high availability of the plant. |
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But: Although short term thermal storage is a conventional technology which is cheap, storage of electricity is still quite expensive. Thus, the optimal matching of BIO-CCHP plant size and subsystems with the demand side system needs to be done to make the most out of e.g. short-term battery storage. Long-term storage is neither relevant for electricity nor heat/cold according to assessment made in this project. However, both short-, medium-, and long term storage needs to be further analyzed in more detail in order to draw any final conclusions and the BIO-CCHP plant needs to be designed and analyzed in terms of how to operate the plant (e.g. part - load operation, flexibility in production capacity of electricity, heating, cooling to match demand side variations). The demand side system also needs to be specified and analyzed for redesigning it to improve energy efficiency and better match the supply side (e.g. switching from electric heating/cooling to direct supply of heat/cold).
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Important: The aim of this project is to move the BIO-CCHP system concept towards TRL 2. This implies that there are a lot of uncertainties involved in the assessments made. Thus, the most important conclusion from the project are the different pathways forward to develop sustainable, cost - efficient and commercially relevant BIO-CCHP - systems. These conclusions are:
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