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WASTE HEAT RECOVERYUSING ORC FOR BOTTOMING IC ENGINE

prof. PhD ALEŠ HRIBERNIK

University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, Maribor (SI)E-mail: ales.hribernik@um.si

Abstract

A bottoming ORC system for harvesting rejected energy of IC engine exhaust gases is presented in the paper. A simple model was developed to estimate the ORC system operating parameters, as well as system investment cost and electricity production cost. A parametric study was performed to study the influence of the main system parameters, such as evaporating pressure, heat exchangers pressure drop, and IC engine exhaust gas temperature and mass flow rate on thermal efficiency and electricity production cost. It was found that the economic viability did not only depend on the thermal efficiency of the ORC system but it is influenced highly by the number and size of applied heat exchangers, and by the pressure increase at the exhaust side of the IC engine caused by the pres-sure drop in the heat exchangers. Increased exhaust gas temperature and mass flow rate can improve the economic viability of an ORC system. Both increase the ORC power much more than the ORC system cost, which results in the reduced electricity production cost.

Key words: waste heat recovery, Organic Rankine Cycle, IC engine

1. Introduction

Industry produces an enormous amount of waste heat which is released either via radiation, cooling fluid, exhaust gas or air. But, even though these heat streams are considered waste, they often contain large amounts of exergy, and would be able to perform work through one of the many waste heat usage technologies.

Technologies to use waste heat from industry can be categorised as passive or active technologies [1]. This de-pends on whether the heat is being used directly at the same or at lower temperature level, or whether it is transformed to another form of energy or to a higher temperature (Fig. 1). Heat exchangers and thermal energy storages are the two dominant passive technologies. These technologies can be used for recycling or reusing waste heat within an industry to heat or preheat other processes. Sorption systems, mechanically driven heat pumps and Organic Rankine Cycles (ORC) are active technologies. Active applications of waste heat are categorised into three types: To provide heat (WHTH), cold (WHTC) or electricity (WHTP).

Fig. 1: Categorisation of waste heat recovery technologies [1]

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Table 1: Exhaust gas temperatures of different processes [2, 3]

Process Exhaust gas temperature (0C)

Iron and steelmaking 1450–1550Nickel refining furnace 1370–1650Steel electric arc furnace 1370–1650Glass melting furnace 1300–1540Basic oxygen furnace 1200Aluminium reverberatory furnace 1100–1200Steel heating furnace 930–1040Copper reverberatory furnace 900–1090Glass oven without regenerator 900–1300Iron cupola 820–980Cooper refining furnace 760–820Reheating furnace without regenerator 700–1200Hydrogen plants 650–980 Fume incinerators 650–1430Coke oven 650–1000Glass oven with regenerator 600–800Cement kiln 450–620Heat treating furnace 430–650Melting oven 400–700Gas turbine exhaust 370–540Reciprocating engine exhaust 320–590Reheating furnace with regenerator 300–600Blast furnace stoves 250–300Drying and baking ovens 230–590Steam boiler exhaust 230–480Finishing soaking pit reheat furnace 200–600Steam boiler 200–300Stack gas 160–200Container glass melting 160–200Flat glass melting 140–160Ceramic kiln 150–1000Drying, baking, and curing ovens 90–230Cooling water from annealing furnaces 70–230Cooling water from internal combustion engines 70–120Exhaust gases exiting recovery devices in gas-fired boilers, ethylene furnaces, etc. 70–230Conventional hot water boiler 60–230Process steam condensate 50–90Condensing hot water boiler 40–50Hot processed liquids/solids 30–230Cooling water from air conditioning and refrigeration condensers 30–40Cooling water from air compressors 30–50Cooling water from furnace doors 30–50

Temperature is one of the most important criteria when considering if the process would either produce valuable waste heat or could use waste heat as an energy source. Typically, high temperature waste heat has more potential to be reused. Fig. 2 shows the distribution of high (HT > 400 0C), medium (MT 100 – 400 0C) and low temperature (LT < 100 0C) applications in different sectors [1]. High temperature waste heat holds a large share of the processes in the metal production and mineral processing sectors. On the other hand, the food and tobacco industry has only negligible amounts of high temperature heat demand. According to Brueckner et al. [1] the processes with an input temperature above 200 0C are to be considered as possible waste heat sources, since their exhaust gas temperature is likely to be at a usable level. Processes with input temperatures below 200 0C, according to their assumption, will only produce waste heat at ambient temperature or little above. In Table 1, industrial processes and their exhaust gas temperature are shown as possible sources. Here, exhaust gas temperatures of different processes from 30 to over 1600 0C are given. Those are not to be mistaken for process temperatures, which will be even higher.

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Fig. 2: Process heat demand by temperature of different sectors [1]

Diverse technologies exist to recover energy in the form of power from waste heat using waste heat as the primary energy source. Examples of technologies for work generation include thermoelectric generators, phase change materials, Organic Rankine Cycles (ORC), Kalina cycles and trilateral flash cycles [4]. In thermoelectric generators, electricity is generated when a voltage difference occurs in a conductor because of a temperature gradient caused by the transfer of thermal energy through the material [5]. Commercially available low-temperature thermoelec-tric materials are up to 250 0C [6]. The generators have no moving parts, are compact, quiet, highly reliable and environmentally friendly [7]. However, relatively low efficiency has limited its use (typically around 5–10%), but they have high capability for utilising huge amounts of waste heat in an easy and simple manner [7]. Phase change materials use the expansion and contraction of a paraffin mixture as it changes from solid to liquid state to produce electricity from heat. Mechanical energy from expansion and contraction is converted into electricity in a genera-tor [8]. The electrical efficiency is very low; 2.5–9% [9] and the technologies are still in the demonstration phase [9]. The Organic Rankine Cycle uses a circulating organic fluid pumped around the circuit and heated by waste heat in the evaporator to produce a vapour which expands to generate electricity [6]. It has received interest in recent years due to its high efficiency and flexibility [10]. Kalina cycles generate electricity from waste heat using a mixture of two fluids with different boiling points [11]. More heat can be extracted from the heat sources compared to some pure working fluids, because the mixture evaporates gradually over a range of temperatures [12]. Organic Rankine Cycles and Kalina cycles can be evaluated by comparison with steam under the same residual condition. Firstly, the ORC has higher thermal efficiency, smaller system volume and weight [13]. Secondly, the Kalina cycle has a better thermodynamic performance. Trilateral flash cycles deliver power by flash expansion of pressurised boiling water, and have smaller thermal efficiencies than Organic Rankine Cycles at the same maximum and mini-mum cycle temperatures, but are still in a state of technical development [14]. Even though the power output for the Kalina cycle is 3% [15] more than the Organic Rankine Cycle, the Kalina cycle has greater cycle complexity and higher capital outlay [16]. Also, this small gain in performance requires a complicated plant scheme, large surface heat exchangers, and particular high pressure resistant and no corrosion materials [17]. The Organic Rankine Cycle is the most mature and tested technology when compared to Kalina cycles and thermoelectric generators [16]. It has a higher conversion efficiency and longer technical life [9]. The Organic Rankine Cycle is suitable for power generation from 30 kW to 20 MW and industrial demonstration projects also exist. Therefore, the present work considers Organic Rankine Cycles. The methodology developed can be applied to other thermodynamic cycles producing electricity from waste heat. The aim of this paper is to show the potential for power generation from available heat in diesel engine exhaust by ORC. A simple ORC simulation model was developed in order to perform a parametric study on the thermal effi-ciency and economic viability of ORC under different thermal parameters (temperature and heat rate) of available waste heat. It was found that the economic viability did not only depend on thermal efficiency, but it is influenced highly by the number and size of applied heat exchangers, and by the pressure increase at the exhaust side of the IC engine caused by pressure drop in the heat exchangers.

2. ORC model

The system layout is presented in Fig. 3. Exhaust gases leaving the IC engine flow through the super-heater, evapo-rator and preheater, and reject their heat to the working fluid before being released to the atmosphere at approxi-mately 120 0C, which is set as the lower limit in order to avoid any water condensation within the exhaust. High

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pressure working fluid vapour expands in the turbine and then enters the regenerator, where the exhausted vapour rejects heat to the vapour cooler integrated within the water cooled condenser, where it finally condenses to the liquid phase. The condensate is then pumped to the working pressure and fed to the system of heat exchangers to produce fresh high pressure superheated vapour.

Fig. 3: ORC model

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5. Conclusions

A bottoming ORC system for harvesting rejected energy of IC engine exhaust gases is presented in the paper. A simple model was developed to estimate the ORC system operating parameters, as well as system investment cost and Electricity Production Cost. A parametric study was performed to study the influence of the main system parameters, such as evaporating pressure, heat exchangers pressure drop, and IC engine exhaust gas temperature and mass flow rate on thermal efficiency and Electricity Production Cost. The following conclusions can be made:

- Electricity Production Cost does not correlate proportionally with the thermal efficiency. A thermodynamically more efficient ORC working with superheated vapour does not attain higher economic efficiency than a simple ORC working with saturated vapour; moreover, the estimated Electricity Production Cost was more than 15% higher. - Pressure drop at the exhaust gas side of heat exchanger can reduce the topping IC engine output power sub-stantially, therefore, special attention has to be paid to holding pressure drop low even at the cost of increased investment cost of the heat exchanger.

- High exhaust gas temperature and mass flow rate improve the economic viability of an ORC system the most. Both increase ORC power faster than system cost. Therefore, the Electricity Production Cost reduces with exhaust gas temperature and mass flow rate.

References

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