Introduction
Designing a small-scale renewable fuel-burning power plant requires careful selection of technology, thermodynamic design, and cost estimation. This sample explores the Rankine cycle application for a 60 MWe island-based biomass power plant, highlighting its efficiency, fuel choice, and economic feasibility.
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1. Selection of Power Generation Technology
Selection of the steam cycle for this small-scale renewable fuel-burning power plant was based on the fact that it is highly efficient, its reliability has been proven, and it can work well with renewable fuels such as wood chips. The Rankine steam cycle is particularly fitted for plants of continuous, base-load power generation and thus ideally suitable for the energy needs of such a remote island of 60 MWe.
Opposite to other alternatives, like gas turbines, diesel engines, and combined cycles, the steam cycle presents higher efficiency when biomass fuels are used. The configuration of the high-pressure boiler and turbine in the plant allows a renewable fuel energy-to-electricity conversion at an overall plant efficiency of 26.27%. The steam cycle therefore represents a practicable solution to meet minimum operation costs with a high level of reliability.
Besides, the steam cycle can be fired with various types of renewable fuels, such as wood chips, biosyngas, or biogas, depending on what is most available locally. This will be very important in the long run, since generating costs for electricity are at a minimum. In addition, the advantages are complemented by low emission from the use of renewable fuels in a steam cycle that guarantees sustainability, with very low specific emissions compared with fossil fuel-based alternatives.
2. Thermodynamic Design of the Plant
Turbine inlet conditions: 500°C, 100 bar
Condenser pressure: 0.1 bar Pump efficiency: 80%
Figure 1: Steam plant Schematic Diagram
CAD design
Figure 2: Boiler Tank
Figure 3: Condenser Tank
Figure 4: Release Valve
Figure 5: Steam Engine
Figure 6: Plant Assembly
Key Thermodynamic States
Mass flow rate calculation:
60 MWe
Output Generator efficiency = 98%
Turbine power output = 60 MWe / 0.98
= 61.22 MW m
= Turbine power output / (h1 - h2) m
= 61.22 * 1000 / (3375.1 - 2373.3) m
= 61.22 * 1000 / 1001.8 m
= 61.11 kg/s
Turbine work output (per kg of steam):
w_turbine = η_turbine * (h1 - h2s) w_turbine
= 0.85 * (3375.1 - 2193.7) w_turbine
= 1001.8 kJ/kg
Pump work input (per kg of water):
w_pump = (h4 - h3) / η_pump w_pump
= (197.9 - 191.8) / 0.8 w_pump
= 7.625 kJ/kg
Net work output (per kg of steam):
w_net = w_turbine - w_pump w_net
= 1001.8 - 7.625 w_net
= 994.175 kJ/kg
Heat input in boiler (per kg of steam):
q_in = h1 - h4 q_in
= 3375.1 - 197.9 q_in
= 3177.2 kJ/kg
Thermal efficiency:
η_thermal = w_net / q_in η_thermal
= 994.175 / 3177.2 η_thermal
= 0.3129 or 31.29%
Total heat input required:
Q_in = m * q_in Q_in
= 61.11 * 3177.2 Q_in
= 194,167 kW
Actual heat input considering boiler efficiency:
Q_in_actual = Q_in / η_boiler Q_in_actual
= 194,167 / 0.85 Q_in_actual
= 228,432 kW
Condenser heat rejection:
Q_out = m * (h2 - h3) Q_out
= 61.11 * (2373.3 - 191.8) Q_out
= 133,318 kW
Overall plant efficiency:
η_plant = (Net electric output) / (Heat input to boiler) η_plant
= 60,000 / 228,432 η_plant
= 0.2627 or 26.27%
T-s Diagram
Component Sizing
Boiler: Thermal capacity = 228,432 kW
Turbine: Power output = 61.22 MW Steam flow rate = 61.11 kg/s
Inlet conditions: 500°C, 100 bar
Outlet conditions: 45.81°C, 0.1 bar
Condenser: Heat rejection = 133,318 kW Condensing temperature = 45.81°C Pressure = 0.1 bar
Pump: Power input = m * w_pump = 61.11 * 7.625 = 466 kW Flow rate = 61.11 kg/s Pressure rise = 99.9 bar
Generator: Capacity = 60 MW Efficiency = 98%
3. Cycle Efficiency Calculation
Mass flow rate (m) = 61.11 kg/s
Turbine inlet enthalpy (h1) = 3375.1 kJ/kg
Turbine outlet enthalpy (h2) = 2373.3 kJ/kg
Pump inlet enthalpy (h3) = 191.8 kJ/kg
Pump outlet enthalpy (h4) = 197.9 kJ/kg
Boiler efficiency (η_boiler) = 85%
Turbine efficiency (η_turbine) = 85%
Pump efficiency (η_pump) = 80%
Generator efficiency (η_generator) = 98%
Efficiency Calculations:
Thermal Efficiency (η_thermal):
η_thermal = (Turbine work - Pump work) / Heat input η_thermal
= ((h1 - h2) - (h4 - h3)) / (h1 - h4) η_thermal
= ((3375.1 - 2373.3) - (197.9 - 191.8)) / (3375.1 - 197.9) η_thermal
= 994.7 / 3177.2 η_thermal
= 0.3130 or 31.30%
Overall Plant Efficiency (η_plant):
η_plant = η_thermal * η_boiler * η_generator η_plant
= 0.3130 * 0.85 * 0.98 η_plant
= 0.2610 or 26.10%
Performance Metrics:
Net Power Output:
W_net = m * ((h1 - h2) - (h4 - h3)) * η_generator W_net
= 61.11 * (994.7) * 0.98 W_net
= 59,580 kW or 59.58 MW
Heat Input to Boiler:
Q_in = m * (h1 - h4) / η_boiler Q_in
= 61.11 * (3375.1 - 197.9) / 0.85 Q_in
= 228,432 kW or 228.43 MW
Heat Rate = Q_in / W_net Heat Rate
= 228,432 / 59,580 Heat Rate
= 3.83 kJ/kWh
Specific Steam Consumption (SSC):
SSC = m / W_net SSC
= (61.11 * 3600) / 59,580 SSC
= 3.69 kg/kWh
Improvement Opportunities:
- Steam parameters: The improvement in steam temperature and pressure at the inlet stage of the turbine can improve the thermal efficiency, but this will render more expensive material for the boiler; hence, maintenance costs would increase.
- Reheat cycle: A reheat stage can be added after the expansion in the high-pressure part of the turbine in order to raise the steam temperature to its original superheat values following partial expansion and thus improve efficiency. This method can enhance efficiency by about 4-5%, but this will add too much complexity to the overall system.
- Regenerative feedwater heating: The use of extracted steam from the turbine to preheat feedwater can raise efficiency by about 3-4%.
- Decrease Condenser Pressure: Decreasing the condenser pressure-if possible, based on the temperature of the cooling water-would result in higher turbine work output.
- Improve Component Efficiencies: More efficient turbines, pumps, and generators would directly lead to higher plant efficiency overall.
4. Renewable Fuel Specification and Cost Estimation
Fuel Specification: Wood Chips
The fuel will be wood chips, a common renewable biomass feedstock that could be supplied sustainably from managed forests on or near the island.
Fuel Properties:
Lower Heating Value (LHV): 10 MJ/kg
Bulk density: 250 kg/m³
Moisture content: 30% - wet basis
Ash content: 1%-dry basis
Fuel Requirement Calculations
From our previous calculations:
Heat input to boiler (Q_in) = 228,432 kW = 228.432 MJ/s
Boiler efficiency (η_boiler) = 85%
Fuel energy input required:
Fuel energy input = Q_in / η_boiler Fuel energy input
= 228.432 / 0.85
= 268.74 MJ/s
Mass flow rate of wood chips:
Mass flow rate = Fuel energy input / LHV Mass flow rate
= 268.74 / 10
= 26.874 kg/s
Daily fuel consumption:
Daily consumption = Mass flow rate * 3600 s/hr * 24 hr/day Daily consumption
= 26.874 * 3600 * 24
= 2,321,914 kg/day ≈ 2,322 tons/day
Cost Estimation
|
Parameter |
Value |
Unit |
|
Heat Input to Boiler |
228.43 |
MW |
|
Boiler Efficiency |
85% |
|
|
Fuel Heating Value (LHV) |
10 |
MJ/kg |
|
Mass Flow Rate of Wood Chips |
26.87 |
kg/s |
|
Daily Fuel Consumption |
2322 |
tons/day |
|
Cost of Wood Chips |
£50 |
per ton |
|
Daily Fuel Cost |
£50 * 2322 |
£116,100/day |
|
Annual Fuel Consumption |
2322 * 365 |
847,530 tons/year |
|
Annual Fuel Cost |
£50 * 847,530 |
£42,376,500/year |
5. Key Component Suppliers
The small-scale renewable fuel-burning power plant shall be in need of credible suppliers for the main components. Main component suppliers for such plants would have a great say in terms of the plants' reliability, efficiency, and sustainability. The following are some identified suppliers of main components.
Boiler:
Valmet is a leading global supplier of boilers for biomass-based power plants. They offer effective solutions to utilize wood chips as fuel in the most efficient way, taking into consideration the plant's strategic focus on renewable energy sources. A key aspect of Valmet's boiler offering is the guarantee of high efficiency at low emissions; this makes its systems ideal for this project.
Turbine:
Siemens Energy turbines are considered workhorses, designed to be tuned for small-scale applications like this 60 MWe plant. Siemens supplies an efficient turbine optimized for the renewable fuels on hand, with operating costs minimized.
Condenser:
General Electric Power-designed condenser is specifically for steam cycles. These are some of the best efficient heat rejection products available in the market that could handle the required cooling load of 133,318 kW at the specified conditions. Condensers from G.E. are super solid and last for quite a time in remote areas.
Pump:
Grundfos' pumps assure efficiency and reliability for industrial water circulation. The high pressure/flow of the Rankine cycle can be sustained by the pumps, hence it would be fit for circulating water from the condenser to the boiler.
6. Cost Estimation of Generated Electricity
|
Cost Component |
Value |
Unit |
|
Capital Expenditure (CAPEX) |
||
|
Boiler Cost |
£20,000,000 |
Lump Sum |
|
Turbine Cost |
£10,000,000 |
Lump Sum |
|
Condenser Cost |
£5,000,000 |
Lump Sum |
|
Pump Cost |
£500,000 |
Lump Sum |
|
Installation & Infrastructure |
£10,000,000 |
Lump Sum |
|
Total CAPEX |
£45,500,000 |
Lump Sum |
|
Operational Expenditure (OPEX) |
||
|
Fuel Cost (Wood Chips) |
£42,376,500/year |
Annual |
|
Maintenance & Operation |
£2,000,000/year |
Annual |
|
Labour & Miscellaneous Costs |
£1,000,000/year |
Annual |
|
Total OPEX |
£45,376,500/year |
Annual |
|
Electricity Generation |
||
|
Net Power Output |
60 MWe |
MW |
|
Annual Electricity Generation |
60 MW * 8760 hours = 525,600 MWh |
MWh/year |
|
Cost of Electricity |
||
|
CAPEX Contribution (Over 20 years) |
£45,500,000 / (20 * 525,600 MWh) = £0.0043 |
£/kWh |
|
OPEX Contribution |
£45,376,500 / 525,600 MWh = £0.0864 |
£/kWh |
|
Total Cost of Electricity |
£0.0043 + £0.0864 = £0.0907 |
£/kWh |
Explanation:
CAPEX: All Major Equipment Costs comprise boiler, turbine, condenser, pump costs, and all infrastructure/installation costs. CAPEX is distributed over a 20-year plant lifetime.
OPEX: The primary operational cost is the cost of fuel. That, however, is followed by maintenance, labour, and all other sundry costs.
Generation of Electricity: Considering that the plant operates for all hours in a year, i.e., 8760 hours/year, the annual production is considered to be 525,600 MWh.
Cost of Electricity: The overall cost of electricity is considered to be £0.0907/kWh since it is an approximate addition of both the contributions from CAPEX and OPEX.
7. Cost Minimisation Justification
The main focus of this design for a small-scale renewable fuel-burning power plant is to try to cut down the cost of generating electricity based on the following perspectives:
The Rankine cycle provides the most efficient steam cycle; hence, there is an assurance of optimum energy conversion of wood chips into electrical output at an overall plant efficiency of 26.27%. Enhancements like an efficient boiler class of 85% and highly efficient turbine provide less fuel consumption hence operational costs are reduced.
- Biomass Fuel Selection: Wood chips are a cost-effective, locally sourced renewable fuel, having a heating value of 10 MJ/kg. Since it is a renewable resource that can be sourced either on or nearby the island, it can keep transport and production costs at an absolute minimum and very significantly lower the OPEX. It is estimated that the cost of this fuel is £50/tonne, giving an annual cost of the fuel at £42.37 million, which is reasonable given the biomass fuel sources are fairly cheap.
- Low-cost components: The plant uses various reputable suppliers for different parts, such as the boiler from Valmet and the turbine from Siemens. It has adequate performance with minimum maintenance, hence reducing the long-term maintenance costs and improving plant availability.
- Balanced CAPEX and OPEX: Capital cost of the plant is amortized over 20 years and provides only £0.0043 /kWh to the cost of electricity, while the operational cost remains low at £0.0864 /kWh. Combining these elements gives a competitive total cost of £0.0907 /kWh.
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