چرخه های زیرین برای تولید انرژی الکتریکی: بررسی پارامتری راه حل های موجود و نوآورانه برای بهره برداری از منابع حرارتی در دمای پایین و متوسط

Many industrial processes and conventional fossil fuel energy production systems used in small-medium industries, such as internal combustion engines and gas turbines, provide low or medium temperature (i.e., 200–500 °C) heat fluxes as a by-product, which are typically wasted in the environment. The possibility of exploiting this wasted heat, converting it into electric energy by means of different energy systems, is investigated in this article, by extending the usual range of operation of existing technologies or introducing novel concepts. In particular, among the small size bottoming cycle technologies, the identified solutions which could allow to improve the energy saving performance of an existing plant by generating a certain amount of electric energy are: the Organic Rankine Cycle, the Stirling engine and the Inverted Brayton Cycle; this last is an original thermodynamic concept included in the performed comparative analysis. Moreover, this paper provides a parametric investigation of the thermodynamic performance of the different systems; in particular, for the Inverted Brayton Cycle, the effects of the heat source characteristics and of the cycle design parameters on the achievable efficiency and specific power are shown. Furthermore, a comparison with other existing energy recovery solutions is performed, in order to assess the market potential. The analysis shows that the highest electric efficiency values, more than 20% with reference to the input heat content, are obtained with the Organic Rankine Cycle, while not negligible values of efficiency (up to 10%) are achievable with the Inverted Brayton Cycle, if the available temperature is higher than 400 °C.

In several applications of many industrial sectors the seek for low-cost electric energy generation and the demand for increasing values of the fuel conversion efficiency are emerging requirements, while in most of the cases the heat demanded by process is not a problem as it can be easily covered with high thermal efficiency boilers. Moreover, in some cases low or medium temperature exhaust heat fluxes may be present in the industrial plant, when fossil/renewable fuel engines or gas turbines are used or when the boilers output heat is not fully exploited. For example, an internal combustion engine can provide exhaust gases at temperature values typically of 300–450 °C, a gas turbine is characterized by exhaust temperature of 400–550 °C and micro gas turbines can give 250–350 °C; other industrial heat fluxes, e.g. exhaust from ceramic desiccant ovens, concrete kiln gas, leather or food industry discharge heat, can provide similar temperature values ranging from 200 °C to 500 °C, depending on the process operation. The available low or medium temperature heat could be profitably exploited by means of a thermodynamic cycle, conceived as bottomer of the heat production process. Typically, in many small-medium size industrial applications, the amount of thermal power discharged by the topping process can be of the order of magnitude of some hundreds of kW, values not compatible with the adoption of superheated water–steam turbine cycles, with complex and multi-level regenerated architecture, typical of large size power plants and granting high efficiency values.

In the framework of the performed thermodynamic investigation, the ORC technology results as the most performing and well proven solution, in order to exploit low/medium temperature heat sources, even if currently it is not yet developed for small scale applications. The selection of adequate organic fluids represents a key factor to maximize the ORC thermodynamic performance which can be significant, as shown in this study, even if a small size (and relatively low efficiency) turbine is used. The Stirling engine, despite its promising performance in terms of efficiency and specific power when operated with hot thermal source (temperature of the order of 800 °C), shows a drastic performance penalization if connected with lower temperature heat sources (200–500 °C) and does not seem the most promising solution to recover a wasted heat flow supplied by a topping industrial process. Among the investigated heat recovery strategies, the innovative and not yet developed IBC system is a promising solution but not as performing as the ORC technology, especially in the field of very low temperatures (200–400 °C). If instead heat fluxes are available at temperature values above 350–400 °C, the IBC technology becomes more interesting in terms of achievable efficiency. Moreover, the IBC system allows a cogenerative operation with thermal power production at the IC heat-exchanger, not investigated in this study. The IBC components are well known and available in other applications and do not require advanced technology (in a first prototype developing phase, the possibility to reutilize in a IBC small turbochargers or microturbine components could be assessed); nevertheless, in order to maximize the system performance an optimized design of the IBC turbomachinery would be required.