Experimental study on a heat driven refrigeration system based 1 on combined organic Rankine and vapour compression cycles

13 Waste heat recovery has been considered as an attractive technique to improve the 14 overall energy utilization efficiency of internal combustion (IC) engines. In this paper, 15 as distinct from most past research work, a thermally driven refrigeration system based 16 on combined organic Rankine and vapour compression cycles is proposed to recover 17 the IC engines’ waste heat contained in the cooling water. Based on the proposed 18 concept, a lab-scale prototype has been designed and constructed using off-the-shelf 19 components to prove the feasibility of producing refrigeration for ships and refrigerated 20 lorries. In this prototype, the power generated by the Organic Rankine cycle (ORC) is 21 used to drive the compressor of a Vapour Compression Cycle (VCC) through a belt 22 transmission mechanism. Pentafluoropropane (R245fa) and Tetrafluoroethane (R134a) 23 are used as the working fluids for ORC and VCC systems, respectively. An electrical 24 water heater is used to simulate the cooling jacket, while a cooling enclosure is used to 25 simulate the cooling load. With the hot water at a temperature around 95 ° C, the system


Introduction
Global CO2 emissions in 2019 still remains at a high level (around 33 Gt) in spite of the rapid increase of renewable power production (mainly wind and photovoltaic) and fuel switching from coal to natural gas [1].In the UK, transport is the largest contributor to its domestic greenhouse gas (GHG) emissions, contributing 28% of its domestic emissions in 2018 [2].The decarbonisation of the world's economy to mitigate the impact of climate change will require us to substantially decarbonise the transport sector.
In most IC engines, around 50-65% of the thermal energy produced by burning fossil fuel is eventually discharged to the environment as waste heat through the engines' jacket water and exhaust gas.Roughly, half of the waste heat is carried away by exhaust gases, and the other half is taken away by cooling water running through the cooling jacket [3].Therefore, it is important to improve the thermal efficiency of engines by means of waste heat recovery, which is considered to be the most promising way to improve the IC engine performance in the next 30 years.
Energy recovery from engine waste heat has attracted considerable academic and industrial research efforts.However, the recovered heat from exhaust gas is normally converted to electricity via different technologies such as thermoelectric generator and organic Rankine cycle power plants and less attention has been paid to the other types of useful output, such as cooling.For vessels, particular fishing boats, refrigeration plants are important auxiliary systems to provide air-conditioning, ice-making, and medicine or food preservation [4], typically powered by separate on board engines or generators.Considering the large quantity of waste heat discharged by IC engines and the vessels demand for cooling, waste heat driven cooling technologies potentially offer an alternative solution.
There are a variety of thermally powered refrigeration technologies including absorption [5,6], adsorption [7], and combined Organic Rankine cycle -Vapour Compression Cycle (ORC-VCC) systems [8].Absorption refrigeration plants have been widely studied and applied to industry and district cooling networks, showing remarkable energy saving benefits [9][10][11].However, absorption chillers are generally used for large-scale stationary applications, due to their higher complexity and large space requirement.Moreover, the coefficient of performance (COP) is generally low for single-stage absorption cycle systems (COP<0.7)using LiBr-H2O pair and even lower for systems using ammonia-water (NH3-H2O) as working fluids [12].Adsorption heat pumps or refrigerators are still at the earlier stage of lab development, and they are unsuitable for mobile applications.
On the other hand, some effort has been devoted to the integration of ORC power plant waste heat recovery system with vapour compression refrigeration systems to develop a new type of heat driven cooling technology.The concept of combining ORC with VCC was proposed as an alternative refrigeration method by Prigmore and Barber [13].The ORC-VCC combined cycle system is an alternative to the absorption cooling cycle, which can provide either cooling or electricity when cooling is not required, increasing the operational flexibility and improving the economic profitability [14,15].
Wali [16,17] compared the performance of solar powered ORC-VCC systems for building cooling applications with five different working fluids.Liang [18] numerically compared two different layouts of ORC-VCC, one of which uses a belt transmission unit and the other is directly coupled using a common drive.Although directly driven units are more compact and reliable, the belt transmission unit results in a better performance since it enables the ORC-VCC to independently operate at their optimal conditions.To study the transient performance of such a concept, Kutlu et al.
theoretically investigated a solar powered ORC-VCC system by considering the offdesign behaviour of the system as a result of natural transient nature of solar energy [19].
To simplify the system structure for downsizing, Aphornratana and Sriveerakul [20] proposed a novel ORC-VCC concept, of which the compressor and expander are integrated in the same unit, using the same working fluid and sharing the same condenser.Bu et al. [21][22][23] carried out a series of investigations on the working fluid of ORC-VCC ice makers and found that n-butane (R600) is the most suitable working fluid.Based on such a system with simple structures and convenient maintenance, Bao [24] carried out performance comparison between using single fluid and dual fluid and concluded the best option for different conditions.
In addition to the above studies, several other researchers [25][26][27][28] also reported the performance of the ORC driven VCC for heating purposes.In our previous study [29], a novel ORC-VCC was proposed for heating purposes.Different from the other systems, the water is heated in two stages, firstly in the VCC condenser at a lower temperature and then in the ORC condenser at a higher temperature.The integration of ORC with VCC in this way enables the utilisation of the low-temperature condensation heat of the vapour compression cycle.
Although there are numerous theoretical investigations on the combined ORC with VCC systems, the system performance was evaluated based some assumption, including fixed efficiency of components, fixed losses and steady conditions.However, the operation of such combined systems will be significantly affected by many factors during the practical operation, some of which can't be ignored or can't be given as fixed value.Therefore, prototyping and experimental research are very important to verify the theory and modelling.A comprehensive literature review shows that the experimental research is scarce, and only one experimental study on the ORC-VCC system was carried out by Wang [14].Their ORC expander and the VCC compressor shared a common drive shaft to reduce energy conversion losses.However, the rotation speed and torque of the VCC compressor are exactly the same as that of the ORC expander.Meaning that the ORC and VCC can't be operated at their own optimal conditions simultaneously because the ORC's optimal condition is decided by the heat source and the VCC's optimal condition is decided by the refrigeration requirement.
In the present paper, a lab-scale prototype of the proposed ORC and VCC system has been designed and constructed using off-the-shelf components, based on which a comprehensive experimental evaluation was carried out to determine the feasibility of producing refrigeration to meet the cooling/refrigeration requirements for shipping by heat recovery of the engine's jacket water.Different from Wang's study [14], a belt transmission unit is used in the present study to change the rotational speed ratio between ORC and VCC to find out the optimal way to connect the expander and compressor as their torque profiles differ each other.Furthermore, the prototype has been tested under both steady state and transient state conditions to understand its dynamic operational characteristics.

Figure 1. Schematic diagram of ORC-VCC combined system
As schematically shown in Fig. 1, a small-scale heat driven refrigeration system that integrates an ORC power plant with a vapour compression refrigerator was designed and constructed using off-the-shelf components.
In the ORC subsystem (the loop with black line in Fig. 1), R245fa is used as the working fluid due to its desired thermodynamic properties, low toxicity, low flammability, and low corrosiveness.The ORC subsystem consists of an oil-free scroll expander with a rated power output of 1 kW, an evaporator (a plate heat exchanger), two condensers in parallel (plate type) and a diaphragm working fluid circulation pump.Cooling water

Hot water
The motor connected to the circulation pump is wired with a variable-frequency invertor, which is used to regulate the flow rate of R245fa.Hot water provided by an 18 kW water heater (the red part in Fig. 2) is used as the heat source to simulate the cooling jacket water of IC engines.The hot water temperature is controlled by a Proportional-Integral-Derivative (PID) controller, which maintains the water temperature at a desired set point.The hot water is circulated by domestic central heating pump, rated to a maximum flow rate of 3.3 m 3 /h.In the VCC subsystem, R134a is used as the refrigerant, which is widely used in various mobile air-conditioning applications.The VCC compressor is connected with the ORC expander via a belt transmission unit, of which the expander-compressor speed ratio can be adjusted by changing pulleys with different sizes.The VCC subsystem consists of an oil-free scroll compressor, a fin-tube evaporator with 3 electrical fans, a thermostatic expansion valve (TEV) and a condenser (a plate heat exchanger).A filter is installed at the receiver tank outlet to remove impurities and a sight glass is installed at the filter outlet to check the state of the refrigerant.A photo of the prototype is shown in Fig. 2. The specifications of the main components of the prototype are listed in Table 1.
The cooling water temperature can be regulated by changing the mixing ratio of cold and hot water steams, varying from 10 to 35 ˚C.As shown in Fig. 1, the cooling water firstly flows through the ORC condenser and then the VCC condenser.The VCC evaporator is placed in an enclosure with dimensions of 1.7m´1.4m´0.8m.The fans circulate the air flow inside the enclosure.To prevent heat leakage from the ambient to the enclosure, it has been insulated using black Nitrile rubber sheets with thickness of 10 mm.Condenser-VCC can be calculated as the following: ( ( (3) ) ) ) The ORC was connected to the VCC by using a belt transmission unit.The mechanical loss through the belt power transmission system is ignored and the compressor power is assumed to be equal to that generated by the expander. ( As the ORC-VCC is essentially a heat driven refrigeration system, the heat-tocooling efficiency is defined to evaluate the system performance of the combined system as: The expander-compressor speed ratio is defined as the ratio of the expander speed to the compressor speed:

Results and discussion
The reading of the thermocouples, pressure transducers, and flow meters are recorded using the data acquisition system at a sampling frequency of 0.2 Hz.Both the steady and transient behaviour of the combined ORC-VCC system have been tested.

Uncertainty analysis
The performance of the system was measured at various inlet temperatures of heat source and sink, when the flow rates of working fluids and cooling water varied.The accuracy of measured parameters listed in Table .1 were considered with the system error propagation.The Kline and McClintock relationship [33] has been employed to ) ( ) calculate the total uncertainty of heat-to-cooling efficiency.For example, the temperature of refrigerant at the ORC evaporator inlet and outlet are 18.2±0.07˚C and 94.0±0.38 ˚C, respectively.The inlet and outlet pressure are 109.94±0.09 PSI and 106.01± 0.08 PSI, respectively.The flow meter of the ORC refrigerant , 0.0403±0.0027kg/s, is also required to calculate the density and mass flow rate of the refrigerant.In this approach, the relative error of heat-to-cooling can be calculated to be around 6.75%.

Steady state test of ORC-VCC
This section discusses the performance of the ORC-VCC prototype operated under partial load conditions at a steady state.The performance is evaluated based on the characteristics mainly of the VCC cycle, the refrigeration temperature, the cooling capacity and the overall heat-to-cooling efficiency.Due to the large amount of experimental data collected, the effects of different operating parameters have been considered, including the mass flow rate of ORC working fluid and the expandercompressor speed ratio.

Effect of ORC mass flow rate
The heat source temperature remains at 94.6 °C.The mass flow rate of the cooling water is kept constant at 0.173 kg/s.The expander-compressor speed ratio is 1.71, using the pulleys with 28 and 48 teeth at expander and compressor side, respectively.Since the teeth on the pulleys are of the same size, the diameters are proportional to the teeth number.For the vapour compression refrigeration subsystem, the compressor speed is commonly used to control cooling capacity and cooling temperature.In the experimental procedure, the compressor rotation speed is controlled by the expander in ORC.Therefore, the effect of the ORC mass flow rate on the system performance under different cooling water temperatures and heat source temperature is studied to explore the interactions between ORC and VCC in this section.Figure 3 shows that the compressor rotation speed increases with the rise of the mass flow rate of working fluid in the ORC subsystem ( !,#$% ).When ORC subsystem is operated at a smaller mass flow rate, the superheat of the working fluid at the expander inlet is relatively higher.As the mass flow rate (  !,#$% ) increases, the pressure difference across the expander increases but the superheat degree decreases.
The maximum pressure difference would appear when the superheat degree becomes 0.
If the flow rate keeps increasing further, part of the working fluid can't evaporate in the evaporator, leading to a decrease in its evaporation pressure.In these tests, the working fluid at the expander inlet is kept within the superheated region.Subsequently, when the mass flow rate  !,#$% was increased by increasing the liquid pump frequency, both the expander intake pressure and the expander rotation speed increase.Since the mass flow rate of the cooling water is kept constant, it is also clear that a lower temperature of cooling water can lead to a higher rotation speed of compressor due to the lower condensation pressure as expected.For the ORC subsystem, the expander will speed up by lowering the cooling water temperature, increasing the compressor rotation speed.Therefore,  !,&%% increases as expected, which can be attributed to the increasing compressor speed and the enlarged TEV opening.significantly.From the perspective of ORC subsystem, the pressure difference across the expander will be reduced if the condensation temperature increases for a given evaporation pressure.As a result, the pressure difference across the compressor also decreases for a given speed ratio since it is driven by the ORC expander.Meanwhile, the VCC subsystem's evaporation pressure P7 decreases since the VCC compressor outlet pressure P8 decreases with the decrease of the cooling water temperature.That is why the evaporation temperature in the VCC subsystem decreases with the decrease of condensation temperature, and the colling load enclosure can reach a lower temperature.
The temperature inside the enclosure will be affected by other external factors (the ambient temperature outside, the insulation material and thickness etc.).The temperature can be maintained as low as -5.6 °C in all the tested conditions when the cooling water temperature is 14.1 °C.unchanged, the cooling capacity will be enhanced by increasing the compressor speed due to the increased flow rate of refrigerant in the VCC and the decreased condensation temperature.This is the reason why the cooling capacity increases with the mass flow rate of working fluid in the ORC subsystem in Fig. 6.Moreover, the variation of cooling water temperature affects both ORC and VCC subsystems.For a lower condensation temperature, at the ORC side, the reduction of the condensation temperature would lead to a larger enthalpy drop of the refrigerant across the expander, leading to a higher power generation.At the VCC side, a higher power transmitted from ORC by the belt transmission unit would increase the cooling capacity in the VCC system.The maximum cooling capacity reaches 1.74 kW under all the test conditions.A sudden drop is shown at the ORC flow rate of 0.028 kg/s.This can be attributed to the fact that the working fluid at the VCC evaporator turns into a two-phase mixture according to the measured temperature and pressure.As a result, some of the refrigerant doesn't evaporate, so less heat is absorbed by the refrigerant.
Figure 7.Comparison of heat-to-cooling efficiency between different heat source temperatures Since this prototype is essentially a heat driven refrigeration system, a heat-tocooling efficiency is used to evaluate the ability of cooling capacity by consuming thermal energy.Fig. 7 shows the comparison of heat-to-cooling efficiency under the same cooling water temperature (Tc=20.5 °C), and two different heat source (i.e., hot water) temperatures Th.It is indicated that the heat-to-cooling efficiency of the ORC-VCC show a similar variation trend with different heat source temperatures, both increasing firstly before reaching a plateau or slight downward trend.According to Eq. ( 6), the heat-to-cooling efficiency is proportional to the ORC thermal efficiency and COPc of VCC.From our previous study on a separate ORC experiment, the thermal efficiency increases firstly and then decreases as the  !,#$% increases, and the peak appears when the superheat degree is around 0. These results agree well with those in Miao [30] and Kosmadakis's [31] study.As shown in Fig. 3, the compressor speed increases as  !,#$% increases.The COPc decreases as the pressure ratio reduces, which is proportional to the compressor rotation speed, as explained in Mateu-Royo's research [22].As a result, the combined effect results in the variation trend of heat-tocooling efficiency as shown in Fig. 7, increasing firstly and then decreasing.

The effect of speed ratio between expander and compressor
From the analysis above, it is found that the load significantly affects the power output, the overall efficiency of expander and generator set of the ORC subsystem.Our original design connects the expander and the compressor using a directly driven shaft, leading to exactly the same rotational speed and torque.However, based on previous calculation results [29], when the ORC and VCC subsystems are operated separately, it is found that the pressure drop across the expander is different from that across the expander when they are operated under the optimal conditions for a given heat source and heat sink conditions.In other words, sharing a common shaft between expander and compressor does not allow that the ORC and VCC to operate under their own optimal conditions simultaneously because their torque profiles mismatch with each other.Therefore, a belt transmission unit is used to study the effect of the speed ratio of expander-compressor on the operation and the system performance.The results in Fig. 8 indicate the compressor rotation speed increases proportionally with the increase of  !,#$% , which seems to be independent from the speed ratio.From the perspective of system operation, the VCC subsystem can be regarded as a variable load of the ORC subsystem.While the ORC subsystem acts as the power source of the VCC subsystem.The variation of load and speed in the ORC subsystem will cause the change of the VCC subsystem's operating conditions.At the same time, the VCC subsystem will feedback such changes to the ORC subsystem through the belt since the compression ratio and speed in the VCC subsystem are dependent on each other.
For the ORC subsystem, the load has significant impact on the power generation, including the rotation speed and the torque output.The output torque of the expander is closely related to the pressure drop across the expander in the ORC subsystem.
During this test, the ORC subsystem's fluid pump is operated at a given frequency.For the VCC subsystem, the rotational speed affects the mass flow rate of the refrigerant significantly, and the torque input is closely related to the compression ratio.As shown in Eq. ( 5), the power consumed by the compressor is delivered by the expander.
Furthermore, the speed at the circumference of these two pulleys is equal to each other as there is no slippage.As a result, the rotation speeds of both ORC and VCC subsystems present the same variation trend, increasing as the mass flow rate of working fluid of the ORC subsystem increases.
Figure 9. Pressures at the expander inlet and compressor outlet with different speed ratios Therefore, when the pulley pairs with higher expander to compressor speed ratios are used, the expander rotational speed will be increased for a given  !,#$% .This can be attributed to the reduced torque output and expansion ratio since the measured evaporation pressure Peva,ORC is decreased, as shown in Fig. 9.
For the VCC subsystem, the increased expander rotational speed would lead to an 1.1 Expander-compressor speed ratio upward trend of the compressor rotation speed.However, the radius of the pulley connected to the compressor is also larger, which would restrain the increase of the compressor rotation speed.Leading to the minor difference of compressor rotational speed while using pulley pairs with different speed ratios.
Figure 10.Comparison of temperature inside the enclosure between different speed ratios For the combined ORC-VCC cycles, there are two important parameters for evaluating the system performance, cooling temperature and cooling capacity.The temperature inside the enclosure, which is closely related to evaporation temperature of the VCC subsystem, is a target temperature according to real applications.In the VCC subsystem, the evaporation temperature is controlled by adjusting the rotation speed of the compressor.It is noted from Fig. 10 that a lower temperature can be achieved by increasing  !,#$% .This is due to the fact that the increasing  !,#$% will increase the rotation speed, which results in higher  !,&%% and pressure drop across the TEV, leading to a lower evaporation temperature as shown in Fig. 10.The lowest temperature in the tests is -3.9 °C when the speed ratio is 1.38.
Figure 11.Cooling capacity of the system when the speed ratio varies Figure 11 indicates that the cooling capacity increases with the increase of  !,#$% .
The cooling capacity is affected by both  !,&%% and the temperature lift of the refrigerant.The reason why the cooling capacity increases with  !,#$% has been explained in Fig. 6.It is also noted that the cooling capacity is a bit smaller when the system is operated with higher speed ratios, although the difference is insignificant.
From the analysis above it can be found that a lower temperature can be realised inside the enclosure when the system operates with a higher expander-compressor speed ratio (shown in Fig. 7).In theory, the opening of TEV would become smaller to achieve a higher pressure difference for a given compressor rotation speed, which will result in a smaller  !,&%% , leading to a smaller cooling capacity in the VCC subsystem.cooling efficiency as shown in Fig. 12.Although the test result fluctuate, we can still tell from the curves that the heat-to-cooling efficiency is lower when the system is operated with a higher expander-compressor speed ratio, which can be attributed to the lower cooling capacity as mentioned previously.

Transient state of ORC-VCC
Due to the direct mechanical coupling between two subsystems, when one subsystem experiences operational change, unsurprisingly it can result in a change to the other.In order to ensure the system can quickly react to regulation and become stable, it is important to investigate the transient response of the whole system.In this section, the temperature and mass flow rate of the cooling water are maintained at 20.5 °C and 0.173 kg/s during the test, respectively.The temperature of the hot water was kept constant at 368K.The results indicate that the evaporation temperature of the VCC oscillates periodically, the TEV maintains the swing in a range of ±1 K around the set value.Generally, the variation trend is decreasing although their peak and trough values oscillate, and finally become stable.In this system, a bulb sensing element is located at the evaporator outlet pipe to control the opening degree of the TEV, which determines the flow area of the TEV according to the feedback from the bulb.The fluctuation of the evaporation temperature results from the variation of the TEV opening degree.The larger the opening, the higher the mass flow rate.
It is well known that when the heat absorbed from the enclosure and the cooling generated in the evaporator achieve balance, the temperature inside the enclosure will become constant.From Fig. 14, it can be noticed that the enclosure temperature (dot line) starts to decrease since the ORC pump frequency rises which steadies out after a period of time.These periods, from the triggering of the working fluid pump frequency to the balanced state, are different.When the system was operated with a higher speed ratio, the enclosure temperature can be maintained at a lower value as the temperature becomes stable.The sustained oscillatory behaviour of the evaporation temperature can be improved by using an electronic expansion valve (EEV), which can perform better since it can achieve a precise regulation for optimal control of mass flow rate even under off-design condition.
Figure 15.Variation of evaporation temperature, pressure and the corresponding superheat degree with response to TEV characteristics, SR=1.12 Interesting results can be found in Fig. 15, which records the evaporation temperature Teva, VCC, evaporation pressure Peva,VCC and superheat degree Tsuper of the VCC when the combined cycle is running under a steady state.Thermal expansion valve (TEV) is typically a linear controller operating simply in response to a change in evaporator superheating.The sensing bulb possess a proportional feedback action control mechanism to keep evaporator superheating at a constant value.When the expansion valve remains unchanged, the differential pressure between the bulb and the evaporation tube equals the spring force.Once the superheating is higher than the set value, the balance will be broken and the opening becomes larger.That is why the  there is no refrigerant R134a flowing through the TEV.Generally speaking, the flat regions at the peak is larger than that at the valley.Meanwhile, the heat-to-cooling efficiency of the overall combined system at the peak is around 0.15.The prototype can have a higher heat-to-cooling efficiency since this is the part load condition of both the ORC and VCC.

Conclusions
In this study, a lab-scale heat driven ORC-VCC refrigerator was constructed and tested under a wide range of operating conditions.The system aims to generate cooling effects by recovering the low-temperature heat contained in the cooling-jacket water.
Several remarks can be summarised as follows: (1) The expander-compressor speed ratio has minor effect on the heat-to-cooling efficiency of the whole system.
(2) A lower enclosure temperature can be achieved when a higher expandercompressor speed ratio is adopted, while its cooling capacity is a bit smaller.
(3) The measured minimum enclosure temperature and maximum heat-to-cooling efficiency is -5.6 °C and 0.18, respectively, under partial load conditions.
(4) The fluctuation of the VCC refrigeration subsystem is mainly caused by opening of the TEV.
In a summary, this research has demonstrated the concept of the ORC-VCC combined cycle for small scale thermally driven refrigeration applications.Further investigation under the design condition will be carried out to optimize system performance in the next steps.The transient performance can be improved by replacing the TEV by EEV.

Figure 2 .
Figure 2. Layout of the ORC-VCC test system

Figure 3 .
Figure 3. Rotation speed of compressor with respect to variation of ORC mass flow rate under different cooling water temperatures ORC [kg/s]

Figure 4 .
Figure 4. Mass flow rate of R134a in the VCC with respect to the mass flow rate of R245fa of ORC

Figure 5 .Figure 5
Figure 5.Effect of the ORC mass flow rate  !,#$% on the temperature inside the enclosure under different cooling water temperatures

Figure 6 .Figure 6
Figure 6.Effects of the ORC mass flow rate on the cooling capacity under different cooling water temperatures

Figure 8 .
Figure 8.Comparison of compressor rotation speed between different speed ratios flow rate of ORC m f,ORC [kg/s]

Figure 12 .
Figure 12.Comparison of heat-to-cooling efficiency between different speed ratios

Figure 13 .
Figure 13.Transient responses of ORC mass flow rate to a sudden increase of ORC circulation pump under different expander-compressor speed ratios

Figure 14 .
Figure 14.Transient responses of VCC evaporation temperature and the temperature

Figure 16 .
Figure 16.Cooling capacity and heat-to-cooling efficiency during transient state

Figure 16
Figure16shows the variation of cooling capacity and its heat-to-cooling efficiency