1. Introduction
At present, increasing global concerns over climate change and energy shortages have resulted in a strong interest in energy development. In China, improving the proportion of natural gas (NG) in the primary energy mix is the main energy conservation and emission reduction strategy in the near future [
1]. Natural gas, which is always used in distributed energy systems (DESs) and combined cold, heat and power (CCHP), will become the major energy suppliers for industrial parks and new towns.
Stationary compressed natural gas (CNG) engines are among of the main consumers of NG. However, most CNG engines in China are developed based on gasoline and diesel engines, hence, the thermal efficiency of most CNG engine is about 30% lower than that of diesel engines due to the restraints of engine structure [
2], that is to say, a large amount of fuel energy (about 35%) is wasted through the exhaust. The conversion of exhaust waste heat into useful output is a promising approach which will improve overall thermal efficiency and save fuel. Previous researchers have shown that among the various techniques for residual energy utilization, the Organic Rankine Cycle (ORC) is a promising method for engine waste heat recovery [
3,
4].
Extensive studies had been conducted on waste heat recovery of internal combustion engines (ICEs) using ORC, including working fluid selection [
5,
6,
7], cycle configuration [
7,
8] and economic analysis [
9,
10]. However, previous studies focused on the design conditions and carry out simulations based on simple thermodynamic models, whereas ORC systems fed by engine waste heat often operate far from their design point due to the changes of engine load. Previous research has shown that the available exhaust gas energy varies greatly depending on engine loads [
8,
11,
12]. In fact, the exhaust gas flow rate and temperature variations lead the evaporator to severe off-design conditions which modify the inlet working fluid conditions until the ORC system becomes unfeasible [
13]. The importance of investigating the off-design performance of ORC systems with variable engine conditions was remarked by Wang et al. [
14]. The stationary ICE is widely used in distributed energy systems (DESs) and combined cold, heat and power (CCHP) systems. The stationary ICE would operate at off-design conditions when the electrical power demand decreases. According to the electricity load profiles of different buildings in a typical day [
15], it is confirmed that the stationary ICE would also operate at off-design conditions for long periods of time. Regarding this, only a few works in the published literature have presented the off-design performance of ORC systems.
Sun and Li [
16] proposed a detailed off-design model of an ORC to predict system performance, and their optimization study revealed the relationship between controlled (optimal relative working fluid mass flow rate, the optimal relative condenser fan air mass flow rate) and uncontrolled variables (the heat source temperature and the ambient dry bulb temperature) on maximal net power output and thermal efficiency. Ibarra et al. [
17] presented a steady-state part-load model for a small subcritical ORC system featuring a scroll expander. The part-load model can simulate the off-design conditions with the variation of four parameters: the maximum temperature of the cycle, the evaporation pressure, the condensation temperature and the expander speed. However, the attention of the authors was only focused to the off-design performance of the expander and pump without considering the off-design simulation of the vapor generator, the heat source and the condensation system. Wang et al. [
18] developed an off-design static model for a solar-powered ORC system and investigated the system behavior with the change of environment temperature, thermal oil mass flow rate and solar radiation. Hu et al. [
19] conducted a detailed design and off-design performance analysis based on a preliminary design of a radial turbine and exchanger based on an ORC system for a geothermal heat source, however, they identified the evaporation and condenser pressure as a control parameters and did not study any parameters affecting them. Quolin et al. [
20] proposed a dynamic ORC model to optimize the working cycle conditions for a wide range of heat source and heat sink conditions. The study of Bamgbopa et al. [
21] extended the findings of Quolin et al. [
20] and set the evaporation temperature as a consequence of evaporator, however, the condenser was simply selected as a unit to complete the cycle at the low pressure side. In summary, the above studies mainly focused on medium-low temperature ORC systems, such as solar and geothermal powered ORC systems.
The exhaust gas of an engine is usually at 500–900 °C which is higher than the decomposition temperature of frequently-used working fluids like R245fa and R123. If the organic working fluids are directly heated by the exhaust gas, it may cause local overheating and decomposition problems. In this case, the medium cycle (MC), an intermediate heat-transfer loop, is generally adopted to lower the exhaust temperature and ensure the safety of the working fluid [
22]. The result of Li [
23] showed that MC can improve the stability of ORC systems and turn all step changes into ramp changes, which makes control system more effective and robust. Gewald [
22] applied an ORC with a thermal-oil cycle to recover the waste heat of several large stationary engines. By using MC, the waste heat of different engines can conveniently be recovered by an ORC system. The importance of medium cycle to transfer heat from exhaust gas to an ORC system was also noted by Vaja [
24]. MC cycles can also be found in [
25,
26,
27]. Previous studies stated that MC cycles can not only inhibit the thermal decomposition of working fluids, but also stabilize the operation of the ORC system under transient conditions. However, the off-design performance of MC/ORC under various engine loads is less studied.
The objective of this paper is to carry out an off-design performance analysis of an MC/ORC fed by the waste heat from an ICE. R245fa is selected as working fluid due to its being non-flammable and non-toxic and having relatively low environmental impact (low ODP and low GWP). The off-design system model is built by assembling the models of each component through the inlet and outlet state. The sliding pressure control is applied to balance the variation of system parameters and evaporating pressure is chosen as operational parameters. The effect of operational variables and engine load on system performance is analyzed from the aspect of energy and exergy to show its maximal working potential.
2. System Description
The internal combustion engine (ICE) selected in this paper is an eight-cylinder four-stroke stationary CNG engine used in a generator set. The main specification of this engine are listed in
Table 1. As the engine runs in a power plant, its speed is constant (600 r/min) while its load varies under different conditions. Seven conditions of the CNG engine are picked out according to the power output range from 400 kW to 1000 kW in intervals of 100 kW. The heat balance of the engine is firstly analyzed according to data from engine tests, as listed in
Table 2. The temperature of the exhaust gas is within 750–813 K and approximately 35% of the fuel energy is wasted in the exhaust gas. Therefore, it is meaningful to recover waste heat from the exhaust gas to improve the engine efficiency and reduce fuel consumption. Under the hypothesis of perfect combustion of natural gas, the composition of the exhaust gas on mass basis has been calculated at: CO
2 = 7.11%, H
2O = 14.22%, N
2 = 73.4%, O
2 = 5.27%, which is used to evaluate the gas properties.
Figure 1 and
Figure 2 show the schematic and the
T-s diagram of the MC/ORC recovering exhaust gas waste heat from the CNG engine. As shown in figure, the red line and blue line represent the exhaust gas flow from the engine and cooling water from the water tower respectively. The black line represents the ORC system. An intermediate heat-transfer loop (the green line) is used between the exhaust gas and ORC circuit to prevent decomposition of the R245fa working fluid. The system operates as follows: the high temperature exhaust rejects heat to the thermal oil and then releases it to atmosphere. The thermal oil supplies the heat to the ORC system in the evaporator (state: a–b). At the same time, the fluid is pumped into the evaporator (state: 7–1) and becomes superheated vapor (state: 1–4). Mechanical energy is produced in the turbine during the superheated vapor expansion (state: 4–5). Finally, it is condensed (state: 5–7) and the next cycle begins.