Massimo Dentice d'Accadia

Thermoeconomic optimization of a refrigeration plant

Abstract In this paper, thermoeconomic theory is applied to the economic optimization of a conventional refrigeration plant, aimed at minimizing its overall operation and amortization cost. Thermal systems cannot always be optimized by means of mathematical or numerical techniques, because a complete model of the plant is not always available, and, in any case, mathematical difficulties are often great, even for not particularly complex systems, and the help of computerized algorithms is needed. In this paper, a simplified cost minimization methodology is applied, based on the so-called Theory of Exergetic Cost, here utilized to evaluate the economic costs of all the internal flows and products of the installation. As shown in the paper, once these costs have been calculated, a design configuration not far from the real global optimum can be obtained by means of a sequential, local optimization of the system, carried out unit by unit, that is, breaking down the global problem into a sequence of simpler problems. In the paper, the case of a very simple plant is considered to develop a numerical example, and, in spite of the approximations introduced to simplify the optimization procedure, the results obtained show acceptable accuracy when compared with those provided by a conventional and more complex optimization methodology.

Thermoeconomic analysis and diagnosis of a refrigeration plant

Abstract An application of the so-called Theory of Exergetic Cost to the analysis of a conventional refrigeration plant is presented. In particular, the study of the effects induced on the overall plant by some malfunctions of its components is performed with the help of the thermoeconomic theory. When a component of a thermal plant displays a deterioration (intrinsic malfunction) not only its performance but also those of the remaining units which make up the structure are affected (induced malfunctions), as they work under operating conditions different from the usual ones. In general, this makes it difficult to understand what would be the effect of any intrinsic malfunction in terms of fuel consumption increase for the overall plant, or, in other words, to determine, for any one of the components characterised by a malfunction, how the overall cost of the final product would decrease if that malfunction were eliminated. Nevertheless, this would allow one to decide, for any one of the devices displaying deterioration, whether restoring its original efficiency is profitable or not. Of course, the exposed problem can be easily solved if a simulation programm is available, but this is not always possible, especially for very complex thermal systems. In this case, the Theory of Exergetic Cost can be successfully applied to the analysis and diagnosis of any malfunction of the system, with no need for complex mathematical tools nor computer simulation. As a first example of thermoeconomic analysis for a refrigeration system, the case of a very simple vapor compression plant is here considered. After proposing an appropriate thermoeconomic representation of the plant, in order to evaluate the exergetic costs of the physical flows which interrelate the structure, an example of application is developed. In this example, a perturbation of an assigned, initial state of the system is considered, caused by given malfunctions of its components; then, for each component displaying malfunction, the overall fuel consumption decrease (Impact on Fuel) achievable by restoring its original efficiency is evaluated by means of the previously calculated exergetic costs. The numerical example shows how the thermoeconomic approach can provide satisfactory results when compared to those of a simulation programm.

Optimum performance of heat engine-driven heat pumps: A finite-time approach

Abstract Classic thermodynamics, assuming the Carnot machine as the upper comparison limit in energy conversion systems analysis, considers thermal equilibrium during heat transfer interactions. Such an assumption requires either infinitely slow cycles or infinitely large heat exchanger surfaces. More realistic limits on the optimal operation of energy systems can be provided by finite-time thermodynamics, which takes into account the constraints represented by finite operation time and limited heat interaction area. This paper focuses on the search for the optimum heating performance of a heat engine-driven heat pump in which the waste engine heat is used for heating purposes. At first, only thermal irreversibilities are considered; then, in order to obtain a more realistic model, the so called “internal” heat leak irreversibilities are taken into account too. Finally, a comparison between the performances of the irreversible model and those of actual plants is carried out.

Optimal operation of a complex thermal system: a case study

Abstract The paper focuses on the search for the optimal operation modes of a complex thermal plant. The system under analysis is basically made-up by four gas-fueled reciprocating engines with heat recovery. Each engine can drive simultaneously an electric generator as well as the compressor of a heat-pump/chiller. The plant is interconnected to the electric utility grid, both to receive additional power and to deliver power in excess. In addition, each heat-pump/chiller can be driven electrically, using the electric generator as a motor. For any given load condition, a large number of operation modes are possible. The problem of finding out the configuration that minimizes the economic cost of operating the system is dealt with. This cost is regarded as the objective function to be minimized in a typical constrained optimization problem. Statement and solution of this problem are discussed. Numerical examples are included and commented.

Field test of a small-size gas engine driven heat pump in an office application: first results

SYNOPSIS The Gas engine driven Heat Pump (GHP) represents a logical alternative to conventional electric space-conditioning machines, even for residential and light commercial applications. Engine waste heat recovery and ability to closely follow the load requirements by continuously modulating engine speed are just some of the advantages offered by GHPs with respect to traditional Electric Heat Pumps (EHPs). This paper describes some of the first results provided by a field analysis in progress on a small-size GHP utilised to match the heating and cooling requirements of an office located in Naples, Italy. The results are in good agreement with expectations, justifying the attention paid world-wide to the GHP technology.

CHAPTER 13:Integrated SOFC and Gas Turbine Systems

Solid Oxide Fuel Cells (SOFC) are particularly promising for their capability to be integrated in complex power plants, showing ultra-high conversion efficiencies. This chapter investigates some possible layout configurations of hybrid power plants including both SOFC and Gas Turbine (GT) technologies. SOFC/GT power plants have been diffusely investigated in literature, from both numerical and experimental viewpoints. Such systems are mainly fed by methane which can be reformed into hydrogen inside (internal reforming) or outside (external reforming) the stack. The majority of SOFC/GT hybrid systems use the internal reforming arrangement which allows one to minimize system capital cost. The steam required to drive the reforming reaction can be supplied by the anode recirculated stream. Alternatively, such steam can be produced externally, using the heat of the exhaust gases. SOFC/GT power plants can operate both in atmospheric and pressurized modes. The atmospheric plants are easier to manage due to the possibility to decouple SOFC and GT operations. On the other hand, pressurized SOFC/GT power plants show higher efficiencies. More complex SOFC/GT configurations are also analyzed, such as: SOFC/HAT turbines, IGCC SOFC/GT power plants, SOFC/GT Cheng cycles, etc. Finally, the chapter also presents the control strategies to be used in the managements of hybrid SOFC/GT power plants, also showing some results of the transient operation of these systems.

Energetic and economic analysis of integrated systems for municipal solid waste management

SYNOPSIS In order to solve the problem represented by Municipal Solid Waste (MSW) management, it is now common to utilise an integrated management scheme. Integrated systems for MSW management are extremely complex. However, they also represent a very interesting potential resource, from both energy and economic viewpoints. Thus, proper planning of the integrated cycle as a whole is of fundamental importance. This paper focuses on the development of a model for the analysis and optimisation of integrated cycles for MSW treatment. This objective is pursued through an assessment of the entire MSW life cycle. An application is also presented, in the form of a case-study of the Italian Region of Campania, which appears to be of particular interest, due to the recent start of a new regional plan for waste treatment, recycling and disposal.