Design

Optimal Design of Cogeneration Systems in Industrial Plants Combined with District Heating/Cooling and Underground Thermal Energy Storage

Description
Optimal Design of Cogeneration Systems in Industrial Plants Combined with District Heating/Cooling and Underground Thermal Energy Storage
Categories
Published
of 15
2
Categories
Published
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Similar Documents
Share
Transcript
   Energies   2011 , 4 , 2151-2165; doi:10.3390/en4122151 energies ISSN 1996-1073 www.mdpi.com/journal/energies  Article Optimal Design of Cogeneration Systems in Industrial Plants Combined with District Heating/Cooling and Underground Thermal Energy Storage Andrea Reverberi, Adriana Del Borghi and Vincenzo Dovì * Department of Chemical and Process Engineering, University of Genoa, Via Opera Pia 15, Genova 16145, Italy; E-Mails: reverb@dichep.unige.it (A.R.); adry@unige.it (A.D.B.) *  Author to whom correspondence should be addressed; E-Mail: dovi@istic.unige.it; Tel.: +39-010-3532921; Fax: +39-010-3532586.   Received: 2 November 2011; in revised form: 28 November 2011 / Accepted: 2 December 2011 /  Published: 6 December 2011 Abstract: Combined heat and power (CHP) systems in both power stations and large  plants are becoming one of the most important tools for reducing energy requirements and consequently the overall carbon footprint of fundamental industrial activities. While power stations employ topping cycles where the heat rejected from the cycle is supplied to domestic and industrial consumers, the plants that produce surplus heat can utilise  bottoming cycles to generate electrical power. Traditionally the waste heat available at high temperatures was used to generate electrical power, whereas energy at lower temperatures was either released to the environment or used for commercial or domestic heating. However the introduction of new engines, such as the ones using the organic Rankine cycle, capable of employing condensing temperatures very close to the ambient temperature, has made the generation of electrical power at low temperatures also convenient. On the other hand, district heating is becoming more and more significant since it has been extended to include cooling in the warm months and underground storage of thermal energy to cope with variable demand. These developments imply that electric  power generation and district heating/cooling may become alternative and not complementary solutions for waste energy of industrial plants. Therefore the overall energy management requires the introduction of an optimisation algorithm to select the best strategy. In this paper we propose an algorithm for the minimisation of a suitable cost function, for any given variable heat demand from commercial and domestic users, with respect to all independent variables, i.e. , temperatures and flowrates of warm fluid streams OPEN ACCESS   Energies 2011 , 4   2152 leaving the plants and volume and nature of underground storage. The results of the  preliminary process integration analysis based on pinch technology are used in this algorithm to provide bounds on the values of temperatures. Keywords: surplus process heat; cogeneration; district heating/cooling; optimal apportionment 1. Introduction The increasing cost of energy and the continual sharpening of carbon dioxide emission standards in the last decades have urged companies to implement energy saving measures in the design of new industrial processes and when retrofitting existing plants. The most significant savings have been obtained through the energy integration of systems of gradually increasing size. Energy integration inside a process is now a well-established, mature technology, mainly based on exergy methods and pinch analysis. Similarly, energy integration across different processes (the so-called total site integration) has led to considerable advances in the overall efficiency by optimally combining energy needs and availability [1–3]. The inclusion of a bottoming cycle to produce electrical or mechanical shaft power has been part of this integration strategy for some time now, as well as the use of waste heat at lower temperatures for commercial and/or domestic heating. Furthermore, the introduction of new engines, such as the ones using the organic Rankine cycle, capable of employing condensing temperatures very close to the ambient temperature, has made the generation of electrical power at low temperatures also convenient [4]. However, this may enter into conflict with the use of lower temperature energy for district heating, which is becoming more and more significant since it has been extended to include cooling in the warm months and underground storage of thermal energy to cope with variable demand. Thus, district heating/cooling has become a key issue in territorial energy planning. Large power stations have had a  pioneering role in the past and are still at the forefront of this area. However, market pressure and the need to comply with more and more stringent directives and guidelines have encouraged mid to large industrial plants, as well as farming enterprises, to consider integration with the surrounding territory. Regional energy integration which includes the optimal integration of industrial, agricultural, municipal and domestic energy sources and demands over a limited area, is presently in an advanced stage of development. Feasibility analyses are routinely carried out and actual implementations have  been made wherever a general agreement has been reached among all the stakeholders involved. The presence of alternative options (electrical power or heat available for territorial energy  planning) requires the introduction of suitable optimisation algorithms to select the optimal strategy. In this article we examine the options available to both managers and design engineers of industrial plants for the selection of the optimal energy management when territorial integration is taken into account. To this purpose we present an optimisation algorithm that considers possible scenarios and the constraints they are subject to for the attainment of technically sound and financially feasible solutions.    Energies 2011 , 4   21532. Results and Discussion 2.1. Energy Sources in Industrial Plants Potential heat sources are all the streams of the process considered that must be cooled. The heat removed from these streams is first used, whenever possible, inside the process to heat other streams which undergo a phase change or whose temperature has to be increased. This heat transfer is obviously limited by the condition that the temperatures of the streams losing energy be greater than the temperatures of the streams to which the energy is transferred. A powerful implementation of this fundamental principle in process engineering is the well known and widely applied pinch technology [5]. The overall heat requirement is first calculated by  plotting the thermal loads of the cold streams on a single graph (cold composite curve) in a temperature—enthalpy plane. If the specific heats remain constant in the temperature range of each stream, the resulting graph is a piecewise linear function with a slope equal to the inverse of the heat capacity of each stream or of a combination of them if temperature ranges overlap. Similarly, the thermal loads of the hot streams ( i.e. , the streams that must be cooled) can be plotted in a similar graph to  provide the hot composite curve. The optimal heat integration can be identified by letting the minimum temperature difference between the curves be equal to a suitably pre-determined ∆ T   (Figure 1). Figure 1. Hot and cold composite curves of a generic process. The corresponding temperature is called pinch temperature and the fundamental law of pinch technology is the requirement that no heat transfer take place across the pinch. Any such transfer would increase by the same amount both the overall heating and cooling requirement of the process. The grand composite curve can be constructed by plotting enthalpy differences between the composite curves at each temperature (Figure 2). The pinch point lies now on the temperature axis and temperature intervals in which hot streams lie above cold streams can be used for the heat integration of the process. The overall minimum thermal requirement and the minimum required amount of cooling water of the process can now be easily identified as the abscissas of the curve at maximum and minimum temperatures.   Energies 2011 , 4   2154Figure 2. Grand composite curve of a generic process. In addition to providing this information, the grand composite curve is a powerful tool that experienced process designers can take advantage of to optimally modify the configuration of the  plant. For instance, in Figure 3 optimal process integration makes it possible to use only low pressure steam, whereas the heat necessary at higher temperatures is provided by internal energy transfer. Figure 3. Process integration makes it possible to use low pressure steam only. Similarly in Figure 4 only part of the heat is provided by high pressure steam. By considering heat requirements at different temperatures the energy transfer is optimally split between the two steam lines. In Figure 5 the use of internal energy transfer and the recovery of the enthalpy content of flue gases makes it possible to dispense with external energy sources. Indeed the flue gas temperature vs.   Energies 2011 , 4   2155 enthalpy curve (which is a straight line if the specific heat is approximately constant) lies entirely above the grand composite curve of the process and consequently it can supply all the necessary amount of heat. Figure 4.  Reduced use of high pressure steam. Figure 5. Recovery of flue gas enthalpy content. Sometimes external work may be a convenient way to raise the temperature at which heat is available. This is often the case when large amounts of heat are available just below the pinch temperature and corresponding amounts of energy are required at temperatures slightly superior to the  pinch temperature (Figure 6). In these cases the cost of using the work of heat pumps can be less (and sometimes much less) than the cost of the amount of heat saved. Obviously the use of available energy inside the process through heat integration has a higher priority than exporting energy outside of it.
Search
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks
SAVE OUR EARTH

We need your sign to support Project to invent "SMART AND CONTROLLABLE REFLECTIVE BALLOONS" to cover the Sun and Save Our Earth.

More details...

Sign Now!

We are very appreciated for your Prompt Action!

x