Final Report: A 10 Kilowatt, Rankine Cycle Agricultural Waste to Energy Conversion Module Utilizing Ultra Micro Turbo-AlternatorsEPA Contract Number: EPD12016
Title: A 10 Kilowatt, Rankine Cycle Agricultural Waste to Energy Conversion Module Utilizing Ultra Micro Turbo-Alternators
Investigators: Frederick, Gary
Small Business: Fluidic microControls, Inc
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2012 through August 31, 2012
Project Amount: $70,091
RFA: Small Business Innovation Research (SBIR) - Phase I (2012) RFA Text | Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Sustainabile Utilization of Biomass
In 2011, there were 60,000 dairy farms in the United States. Of these, 56,600 were in herds of fewer than 500 animals. The average herd size is 115 cows with 77 percent of dairy farms having fewer than 100 cows (The US Dairy Industry, A Vital Contributor to Economic Development, Dairy Management Inc.). Currently, digester installations are only considered economical for herds of at least 500 animals (Market Opportunities for Biogas Recovery Systems at US Livestock Facilities, AgStar EPA, November 2011). These require turbine or diesel generator sets of 100-kilowatt size and larger. This project evaluated the feasibility of an electrical-power-generating system based on a steam Rankine cycle, sized and designed to burn untreated digester gas from small animal herds. The system also was designed to utilize modified, low-cost, steam boilers for residential heating systems. The steam from the boiler powers a unique turbo-alternator spinning on fluidically-damped gas bearings, also energized by system steam. A schematic diagram of this concept is shown in Figure 1.
Analysis of the Rankine cycle with water as the working fluid was performed. Cycle efficiency was calculated for various boiler pressures, condenser temperatures, degrees of feed water pre-heat and degrees of super-heat. It showed that ideal cycle efficiencies could vary from 31 percent at 160 Psia with no superheat to nearly 40 percent at 400 Psia with superheat. A 10-kilowatt electrical output required 120 Lb/Hr of steam and approximately 50 cows for input power.
Analysis and conceptual design of a steam turbine and permanent magnet alternator for generating electrical power also was completed. The micro-turbo-alternator was designed to operate at high RPMs needed for efficiency, made possible by riding on Fluidic microControls, Inc.'s (FmC) proprietary fluidically-damped gas bearing. The turbine was a two-stage axial impulse type with DeLaval nozzles to allow efficient operation with the high energy steam. FmC incorporated the turbine blades on the outer annulus on a face of the turbine wheels rather than on the outside of the wheel rim as is more conventional. This makes for more efficient, direct impingement of supersonic jets from the nozzles, and allows for high-volume manufacturing processes, such as 2.5D shaping utilizing micromachining, stamping, and etching processes for both turbine blades and nozzles. The alternator was analyzed to show the capability to produce 10 kilowatts of 3-phase power with 95 percent efficiency. The rotor was a two-pole design with samarium cobalt magnets and a 1.3 inch outside diameter. The stator has 12 slots and a 3.7 inch outer diameter.
A key achievement was completion of analysis and testing which demonstrated that the fluidic bearing could operate on the same steam pressure integral to the system, allowing a completely closed fluid loop with no need for seals or external air supplies. The bearing was redesigned to maximize damping at typical turbo-alternator speeds and to minimize flow consumption by utilizing second stage turbine inlet pressure as supply in a unique system configuration, reducing bearing energy consumption to less than 2 percent of system flow. Representative fluidic bearings were spun at over 100,000 RPM on a 0.8 inch diameter shaft when driven by a 1.6 inch diameter impulse turbine using 12 Psig air. A steam test rig was fabricated using a residential boiler, Model EG-30 provided by Weil-McLain. The same bearings were then repeatedly spin tested on 12 Psig wet steam.
A conceptual design for a high pressure, water tube boiler with evaporating, superheat, and condenser water pre-heat heat exchangers and integrated solid-state sensors and controls was completed. It also included an open combustion burner system, which can use combinations of untreated dirty bio-fuels and natural gas.
A costing analysis was performed to estimate system price in 1,000 and 10,000 unit annual quantities. The capital cost per kilowatt goal was $500 compared to greater than $1,000 for micro-turbine and IC engine gen-set installations. The analysis showed a price of $2,769 for the turbo-alternator and $2,910 for the boiler, near FmC's price goal in the larger quantities.
Installation studies showed that this system has many advantages:
- The single module designed for small bio-waste streams may be readily paralleled for higher power system requirements.
- Digester gas may be routed to multiple farm sites to point of use, simplifying installation.
- A common inverter module may be utilized, which also serves other on-farm alternative energy sources, including wind power.
- No significant digester gas pre-treatment or compression is required.
The analyses and studies conducted in Phase I showed feasibility of the steam Rankine system and fluidic bearing turbo-alternator approach for both residential and agricultural applications:
- A Rankine cycle system may be practically scaled to herds with as few as 25 cows.
- The reducing turbines may be suspended on fluidically damped gas bearings operating on system steam for efficiency, simplicity, reliability and long life.
- Electrical power generating efficiency is greater than 20 percent with 30 percent considered feasible.
- Combined Heat and Power efficiency is greater than 90 percent, with 95 percent considered feasible.
- Price of a CHP system with boiler and turbo-alternator of less than $500/kilowatt is achievable.
With 50 cows being able to produce 10 kilowatts of power, there is a market for 90,000 10-kilowatt digester gas to electrical power modules for herds of 500 cows or fewer. FmC's agricultural CHP unit may be readily scaled to provide all the power needs for an average home and simply modified to also burn natural gas. Total energy efficiency of more than 90 percent is achievable for a residential micro-CHP system versus 60 percent being typical for a home buying commercially generated electrical power and using a boiler/forced air heater powered by heating fuel. With low natural gas prices, the micro-CHP system looks even more economical. In addition, the improved overall efficiency reduces CO2 emissions by 30 to 60 percent. 30 to 50 million homes in North America are candidates for CHP. The installed system growth rate over the last 10 years is more than 20 percent, and more than 90 percent of homeowners are interested in installing CHP. The market will continue to grow according to Pike Research (Residential Combined Heat and Power Market to Reach $41 Billion by 2022, Forecasts Pike Research, Business Wire, Feb 23, 2012).
With FmC's unique steam turbo-alternator spinning on steam-powered fluidic bearings, teaming with the right residential heating partner who will also service the agricultural market, could make FmC a major player in CHP systems. The high volumes of the residential market will allow a great reduction in manufacturing costs, allowing FmC to meet its $500/Kilowatt price target for CHP modules for small digester systems and opening up a digester CHP module market of $450 million.
FmC plans to manufacture all or part of the turbo-alternator, and our heating system partner will manufacture the boiler and be responsible for sales and service.