Compact Heat Exchangers for Microturbines

With distributed power generation market, the most economical solution today is to generate power through small gas turbine systems, arbitrarily categorized as microturbines (5 – 200 kW) and miniturbines (200 – 500 kW). The thermal efficiency of such microturbines is about 20% or less if no recuperator is used in the system. Using a recuperator (regenerator can also be considered but has a number of problems) operating at 87% effectiveness, the efficiency of the gas turbine system increases to about 30%, a substantial performance improvement. However, cost of the recuperator is about 25 – 30% of the total power plant. This means that the heat exchanger (recuperator) must be designed to get high performance with minimum cost. While the offset strip fin geometry is one of the highest performing surface it is also quite expensive to manufacture. This necessitates the use of all prime surface heat exchangers with no brazing. In this paper, after providing the necessary concise information on microturbines, the discussion is presented on various types of heat exchanger surfaces and novel designs considered for the cost effective heat exchangers and packaging in the system. For hot fluid inlet temperature of less than about 675°C, stainless steel material can be used for the heat exchanger, which has reasonable cost. However, for higher inlet temperatures in heat exchangers associated with higher turbine inlet temperatures, superalloys are essential which increases the material cost of the exchanger alone by a factor of 4 to 5. The design, material/finished heat exchanger cost, performance, durability, and other related issues of compact heat exchangers for microturbines are covered in this paper. The discussion and coverage is primarily for metal heat exchangers since the ceramic heat exchangers are in infant stage after last 50 years of development associated with the gas turbine applications.