Comparative study of alternative ORC-based combined power systems to exploit high temperature waste heat
Introduction
Energy conservation and emission reduction have always been topics of great concern. As the main source of motive power, internal combustion engines (ICEs) constitute a large proportion of global fuel consumption. Meanwhile, a large amount of fuel energy is released to the ambience in the form of exhaust without effective utilization. In general, it represents approximately one third of energy generated from fuel combustion [1]. If this part of waste heat could be exploited effectively, fuel utilization efficiency for ICEs would improve dramatically, thus alleviating the problem of energy shortage and producing fewer emissions at the same time. Therefore, a number of researchers have showed their interests in this field and many techniques are proposed [2], [3], [4], [5]. Among them, organic Rankine cycle (ORC) shows great potential for its desirable thermal efficiency, low maintenance requirements, and high reliability [6], [7]. Focuses have been mainly given on the working fluid selection [8], [9], [10] and ORC configurations [11], [12], [13], including regenerative ORC, preheat ORC, etc. Most of them belong to one-stage system. However as a matter of fact, ICEs can provide high temperature exhaust gases ranging from 300 °C to 600 °C, depending on the engine operating condition, which is beyond most organic working fluids’ decomposition temperature. Hence, it may pose a threat to the system safety. To avoid this, in some situations, a thermal-oil circuit is added between exhaust gases and ORC system so that waste heat is transferred to the working fluid via thermal-oil [14], [15], [16]. Even though it allows ORC to work at a suitable temperature under the premise of ensuring safety, the great majority of high-grade exhaust heat is not exploited at all.
In the case of ORC for high-temperature waste heat recovery, such as engine exhaust heat, recovering this part of energy completely, and in the meanwhile, preventing the working fluid from decomposition is a problem. For this reason, another technique is proposed to establish combined system with ORC to extend the temperature range of heat source and produce additional power.
The steam Rankine cycle (RC) is environmentally friendly as water has no negative effects on the environment, moreover, water can bear high temperature and it is cheap and abundant. BMW [17] designed a turbosteamer system (i.e. dual-loop Rankine system) which used water and ethanol as high- and low-temperature Rankine cycle working fluids respectively to recover waste heat from exhaust gases and jacket water. Further researches [18], [19] indicate that DORC has great potential for waste heat recovery.
Unlike ORC, Brayton cycle (BC) regards gas as working medium which suits high temperature operation well and it has some interesting advantages over Rankine cycle, such as simplicity and reliability, because it does not involve phase change during the whole process and can work at low pressure ratios [20]. Previous studies [21], [22] have explored its application in waste heat recovery, and air is considered as working fluid. However, primarily limited by the compression consumed power, its performance is not always desirable. While on the other hand, CO2 Brayton cycle becomes appealing for its high ratio of turbine work to compressor work [23], and many scholars are engaged in this field, mainly for nuclear and solar power applications [24], [25], [26]. In such situations, the temperature of heat sources can come up to 600 °C or even more. Chen et al. [27] investigated CO2 power cycle to utilize the energy in the exhaust gases. As a feasible recovery technology, CO2 Brayton cycle is also included in our study for comparison.
Thermoelectric generator (TEG) is another promising technology in the area of waste heat recovery [4], [28], [29], and it has drawn great attention with the development of thermoelectric materials which leads to significant promotion in conversion efficiency. It can convert thermal energy into electricity directly without any moving component. Apart from that, it is also attractive for its simple structure and great flexibility. Miller et al. [30], [31] first proposed the idea of combining TEG with ORC, and TEG did not only convert high temperature exhaust heat into power, it preheated ORC working fluid as well. The detail mathematical model of such combined system is established and discussed in other researches [32], [33].
In this work, our main objective is to provide an adapted solution to exploit exhaust waste heat from ICEs, and three different combined systems which regard ORC as bottoming cycle are evaluated, i.e. dual-loop organic Rankine cycle (DORC), CO2 Brayton–ORC (BC–ORC) and TEG–ORC. Comparative investigations are performed on some key indexes, namely net output power, thermal efficiency, recovery efficiency, exergy efficiency, the turbine size parameter (SP) and heat transfer capacity (UA), thus providing comprehensive information. Besides that, five typical engine conditions are picked out to evaluate the performance of these combined systems on the whole.
Section snippets
ICE system
The engine we analyzed here is an inline 6-cylinder 4-stroke supercharged diesel engine. In our present study, as a commercial diesel generator set, the engine’s speed remains constant at 1500 rpm while its load changes under different working conditions. Five typical engine conditions are chosen and their main parameters are collected in Table 1. Normally, the engine works under the condition 2, i.e. rated condition. Throughout these five conditions, exhaust temperature is between 693 and 808 K.
Mathematic modeling
In order to assess the energy exploitation potential of these identified solutions, numerical parametric investigations are conducted. Specifically, the simulation and modeling is established based on EES (Engineering Equation Solver) [45]. This software is widely used for the merit of being able to invoke various working fluids’ thermodynamic property parameters.
As a common assumption for all considered systems, the minimum temperature of the cold source (cooling water) Tmin is set to be 20 °C.
Model validations
In this section, error analysis is conducted by comparing the present results with previous researches for each model to make sure that the simulation results are reliable and accurate.
Parametric optimization for each combined system
Since the power system is driven by waste heat, the increased power and efficiency come free of charge. The output power is considered as the most important evaluation index for comparing combined systems’ performance. It is a direct indicator for the improvement of fuel economy. Previous researches [51], [55], [56] have been performed for its promotion and maximization.
For DORC, to maintain a consistent heat transfer in the heater H1,LT (cooler CHT), the maximum temperature (evaporating
Net output power (Wnet)
According to the above optimization, the optimum performance in terms of net output power is obtained as shown in Fig. 12. Throughout the all five conditions, DORC shows the highest capacity to generate power, in particular, the Wnet,all is 32.63 kW under the condition 2 (rated condition), accordingly, providing a 5.57% improvement in fuel economy. Followed by BC–ORC system, whose Wnet,all is 1.5–5 kW lower than the corresponding DORC under the identical engine condition. Resulting from the
Conclusions
Aimed to exploit high temperature exhaust heat from internal combustion engine completely and get the maximum power possible, thus achieving significant fuel economy promotion, three combined power systems which regard ORC as bottoming cycle are proposed. The alternative recovery topping cycles are steam Rankine cycle, CO2 Brayton cycle and TEG. Power based optimization is conducted under five typical engine conditions. Based on energy utilization parameters (output power, thermal efficiency,
Acknowledgments
This work was sponsored by the National Natural Science Foundation of China (NO. 51206117), and the National Basic Research (973) Program of China (NO. 2013B707201) and the Natural Science Foundation of Tianjin (NO. 12JCQNJC04400).
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