Heat Recovery: Introduction
A significant portion of fuel oil used in rural Alaska is for space heating. Recovery of wasted heat from diesel power generation has great economic potential for remote Alaskan communities. The recovered heat can be used for space heating, domestic hot water, or for tempering municipal water supplies to prevent freezing and facilitate treatment. The efficiency of recovering waste heat for augmenting electrical power production is lower than that for heating; however, it can be attractive and economical in some places since electrical power is needed all year, and heat is only needed in the very coldest months.
Heat recovery may use one or all of the diesel generator’s waste heat sources including the exhaust stack, jacket water, and charge air. Waste heat recovery using jacket water heat and/or charge air heat directly for heating is a mature and proven technology. Over a quarter of rural village diesel generators have already been equipped with jacket water heat recovery systems. Charge air heat has been recovered for heating in a select number of communities.
- Water Jacket Heat Recovery:For rural Alaska, the technologies systems that use recovered heat directly are most applicable. Modern high-efficiency heat exchangers, super-insulated heat piping, high efficiency electric pumps, modern electronic BTU meters, and variable speed radiator fan motor controllers maximize the utilization of heat available from diesel engines. Waste heat recovery for space heating is a common, proven design. The associated design and maintenance procedures are well understood in the Alaskan power industry. For this reason, water jacket heat recovery for space heating is considered a mature technology in Alaska.
- Exhaust Stack Heat Recovery: Heat recovery from diesel engine exhaust stack is a proven and cost-effective technology in larger power plants. Recent technological improvements have made exhaust stack heat recovery feasible and economical in midsize engines, which are used in rural Alaska. These advances in exhaust stack heat recovery have boosted recovered heat and reduced the hazards and maintenance burdens typical of the older systems. At this time, only one production diesel generator in Alaska, apart from the University of Alaska diesel test bed, is known to employ an exhaust stack heat recovery system for heating applications. This is a relatively large 5 MW power plant at a mining site. No heat recovery performance data for that installation was readily available for this publication.
Diesel stack exhaust heat recovery has high capital and maintenance costs, as well as the potential for excessive exhaust system corrosion and soot build up. The risk of the heat recovery system causing generator failure and higher maintenance costs often outweighs the value of recovered energy. These factors are among the reasons, stack heat is not usually used in rural Alaska. However, advances in exhaust heat exchanger design and operational strategies have reduced the probability of corrosion and soot problems and the low sulfur fuel oil mandate will also reduce corrosion risk, potentially making this technology more appropriate in the future.
Recently, the University of Alaska Center for Energy and Power conducted an experimental study to investigate the economic effect and feasibility of employing exhaust heat recovery techniques on a midsized diesel engine. Based on study results, the diesel exhaust heat recovery appeared to cause no critical problems to engine performance nor to appear to increase maintenance issues. The payback time for this recovery system is estimated to be less than three years at $3 per gallon fuel prices, with engine operation of eight hours per day. Study results and performance of existing exhaust heat recovery systems on large diesel engines in industrial level applications show exhaust heat recovery to be a mature and proven technology, ready for adoption. However, performance and economic results will differ for each project. Influential factors, include power plant load pattern, heating load characteristics, and existing heating system infrastructures. To address these concerns, analyzing the specific generating system to be retrofitted, before the installation of an exhaust heat recovery system is essential. Currently, it is recommended that diesel generator capacity exceed 400 kW and that the community have a year-round population above 700 residents before stack heat systems are installed in rural Alaska.
- Heat to Electricity Technology: There are technologies that allow for waste heat to be converted to electric power. These are the organic Rankine cycle (ORC), Kalina cycle, exhaust gas turbine, and direct thermoelectric conversion systems. For years, engine heat recovery for power generation has been applied to very large power plants and marine engines. Many heat recovery power systems have capacities over a megawatt, including combustion engines powered by natural gas, coal, and petroleum-based fuels.The performance of waste heat to power systems is relatively sensitive to exhaust temperature and the energy content of the heat sources. For mid-sized engines, technologies for converting waste heat to electrical power are not yet considered mature technologies. Feasibility of these systems is highly dependent on fuel cost. Current research and development groups include engine manufacturers and power plant companies. The University of Alaska Center for Energy and Power is also assessing the performance of heat-to-electricity technologies from several manufacturers.
Most existing ORC systems are used in geothermal applications and range in size from about 250 kW to multi-MW. ORC systems for engine waste heat applications have similar capacity. Commercially tested Kalina cycle systems are not common, with only a few in production and almost all of the units in multi-MW capacity. Successful Kalina cycle systems are much larger than a megawatt and would not necessarily scale down to be effective systems on the midsized diesel engines employed in rural Alaska. For many years, small scale, organic Rankine cycle systems were used successfully in some Trans Alaska Pipeline systems. The manufacturer presently produces only large scale systems. Kalina cycle systems based on ammonia are rare, and commercially available options are not of a scale suitable for Alaskan village generators.
The performance of the thermal-to-power conversion systems is sensitive to the properties of heat source, heat sink, working fluid, and energy intensity. For example, resources with similar power capacities may require different systems in order to obtain optimum system performance. Therefore, each prospective installation site will require an individual analysis to insure appropriate operation.