Water Journal : Water Journal May 2011
refereed paper resource recovery water MAY 2011 99 The Capstone Micro-turbine (Figure 2) is a simple elegant system incorporating a recuperator, combustor, turbine and a permanent magnet generator. The rotating components are mounted on a single shaft, supported by patented air bearings that are spinning at up to speeds of 60,000rpm. There is only one moving part in the turbine and it is simply cooled by inlet air. The system uses no oil, no lubricants and no coolants, and has no pumps, gearbox or other mechanical subsystems other than air bearings. Furthermore, the Capstone Micro- turbine technology requires minimal maintenance in terms of replacement parts, as engines are overhauled after a period of nine years and all other parts exposed to the biogas are suitable for hydrogen sulphide (H2S) levels of up to 60,000ppm. It has rapid start-up and shut-down and can be controlled to follow instantaneous power demand. It is slightly less efficient than a diesel- electric set in producing power, but in cogeneration mode the heat is not wasted. Present Value and Choice of Technology Western Water conducted a thorough review of the required thermal energy needs for the plant and biogas quality to establish a clear understanding of the tender requirements. A key learning throughout the process has been that the specific choice of technology is governed by the needs of the particular application. In the Western Water example we required less thermal output than other technologies could have provided, but we did require an equally high power production given the number of substantial power requirements required on-site for aeration, recycled water and other energy-intensive equipment necessary for advanced waste treatment. When providing the present value analysis (Figure 3) Western Water concluded the high availability per annum and low projected maintenance costs of the Capstone Micro- turbine proposed by Aquatec-Maxcon, together with the power produced, made it the best option. Hence, this did not just take into account energy efficiency but the per annum availability of the Capstone equipment, which is well over 99% when compared to other cogeneration technologies available in the marketplace. Typically, the only down time would be for servicing and Capstone estimate six hrs/yr per turbine per year. The Capstone Micro-turbine can operate for 8,000 running hours before the air and fuel filters need to be changed, and fuel igniters and actuators inspected. Power Efficiency The Micro-turbine can be set up to follow the site peak load as it varies during the day, providing power in excess of the base power supplied by the utility grid. The site power load can vary between 240kW and 630kW, depending on the operation of recycled water plant and the number of aeration blowers used for the wastewater plant. This allows the Micro-turbine to track electrical loads at the plant and supply only as much power as required. The Micro-turbine currently supplies between 100kW to 200kW of site power above the base load. This is achieved in either of two ways: • Use all the available biogas in the turbine (when there is sufficient volume in the gas holding tank) to produce 200kW of power and minimise electricity import from the power authority. Due to the small size of the existing gas holding tank there is not sufficient capacity to allow the Micro-turbine to operate for long periods at 200kW; or • When gas availability is low in the holding tank, the Capstone is run at half-capacity, i.e. 100kW, and turned off as necessary if the holding tank runs out of biogas. The Micro-turbine is expected to operate at full demand in the future once Figure 2: The Capstone Micro-turbine. Figure 3: Present Value Assessment Option Aquatec-Maxcon Capstone CR200 Period of Assessment 20 years Discount Rate 5.8% Annual Indexing (%) 2.5% Depreciation 2.5% Tax Rate 30.0% Assumed Power Cost $0.19 per kWhr Capital Expenditure $1.6M Operational Expenditure (over 20 years) (including media replacement for the carbon, engine planned maintenance and overhaul of the turbine every 9 years) $1.6M Cost Offset Due to Power Savings (including projected reliability) ($8.3)M Present Value (After Tax) ($1.1)M (nett saving) Comparison to a Diesel-Electric Genset Option (using similar planned maintenance and recommended engine overhaul periods) $0.5M (nett cost) Figure 4: The Micro-turbine operating at different power settings.
Water Journal April 2011
Water Journal July 2011