Teaching the theory and realities of 2nd law heating systems

Abstract

In response to concerns about global warming by CO2, and depletion of non renewable fuels and need for jobs, there has been increasing interest in teaching renewable or so called Green Energy technologies. They are typically defined as wind, solar and bio-fuels. However, the most potential exists where the most is wasted. The author asserts this is the non-obvious home furnace or heating systems. One reason the home furnace is overlooked is some furnaces are advertised to have a 96 % efficiency. It is notable that prior to the last generation, the best residential furnaces were typically 80 % efficient. This meant that 20 % of the heat went up the chimney in the form of elevated temperature combustion products and water vapor. The step improvement to 96 % efficiency was obtained by increasing the heat recovery surface area with a secondary heat exchanger, which cooled the combustion products down close to space temperature and also condensed most of the vapor in the combustion products. The chimney was replaced with a clothes drier type vent to the side of the house. Thus, virtually all of the sensible heat and most of the latent heat in the combustion products was recovered. The result has been called a condensing furnace, which was commercialized in the mid 1980s. Chimney furnaces need the lower density of the elevated temperature combustion products to cause them to go up the stack by free convection. The cooled combustion products of a condensing furnace are not buoyant and thus require forced purging. This has been achieved in some furnaces by pulse combustion with tuned valves, or alternatively by a conventional combustion chamber and a motor driven fan to draw the products from the furnace. This near 100 % efficiency suggests there is very little room for improvement. However, thermodynamic students should recognize this is a 1st law efficiency. It is the ratio of space heat delivered to the heat value or the amount of heat produced by burning the fuel. If the 2nd law is considered it is recognized that heat must be transferred over a very large temperature difference between the combustion products and the space temperatures. This results in a large entropy production and wasteful irreversibility. It also means a lost opportunity to convert the high temperature heat mechanical power or electricity, that are highly refined forms of energy much more valuable than low temperature heat. An understanding of the 2nd law is needed to explain why a 1st law efficiency of over 100 % is possible. The explanation can be challenging. It should start with defining an ideal heating system, which would have no entropy production and thus no irreversibility. This means determining the minimum amount of fuel to produce the required space heat. © 2012 American Society for Engineering Education.