abstract an experimental study is conducted to investigate the performance of a low

7/31/2019 ABSTRACT an Experimental Study is Conducted to Investigate the Performance of a Low

1/15

1 | P a g e

ABSTRACTAn experimental study is conducted to investigate the performance of a low-temperature solar

recuperative Rankin cycle system using working fluid R245fa. The experimental setup

consisted of typical Rankin cycle components, such as throttling valves, working fluidpumps, air cooled condensers, and a flat plate collector for gathering solar energy. Prior to

boiler, the working fluid is typically preheated; this process is simulated in the experiment

using an electric tracing ribbon. Experiments conducted during typical winter weather

conditions in Tianjin, China. Results showed that for conditions of constant working fluid

flow rates, the introduction of a recuperator did not improve the thermal efficiency of the

experimental system obviously, which remained constant at about 3.67%. Furthermore,

preheating caused the collector inlet temperature to increase, which led to lower collector

efficiencies and ultimately, lower overall system efficiencies. Results also showed that both

thermal and collector efficiencies could be improved significantly by adjusting the working

fluid flow rate to an appropriate level based on the solar heat flux. Significant improvements

in thermal efficiency can also be achieved with an improved expander. Experimental works

on solar powered recuperative organic rankine cycle system is performed using R245fa.With

specific constant flow rate , efficiency of recuperative ORC cannot be improved. Flow rate

has great impact on collector efficiency and rankine cycle efficiency. Combining both

recuperative and flow rate regulation can improve the performance greatly. The cycle

performance for conditions of constant flow rate and variable flow rate are compared,

examined, and discussed.

Keywords: Solar energy ,Low-temperature,Rankine cycle,Recuperative,Working fluid

REFERENCES: J.L. Wang, L. Zhao , X.D. Wang Department of Thermal Energy and

Refrigeration Engineering, School of Mechanical Engineering, Tianjin University, No. 92,

Weijin Road, Tianjin 300072, PR China Received in revised form 12 December 2011

Accepted 7 January 2012


2/15

2 | P a g e

1.INTRODUCTION

Electricity generation from renewable energy sources such as solar, wind, biomass, and

geothermal, have continued to garner more attention because they release very little, if any,

pollutants or agents that contribute to global climate change, making renewables very

attractive environmentally, especially compared to fossil fuels. Solar thermal energy, in

which a heat engine is used to convert collected solar radiative heat into electricity, has

shown potential for electricity production on both a large centralized scale and a smaller,

local scale. To improve solar thermal to electricity conversion, many researchers are

investigating the development and design of new solar thermodynamic cycles and improvingexisting one. Zhanget al. proposed and built a solar energy powered Rankine cycle using

supercritical CO2 for combined production of electricity and thermal energy in Japan [58]

For the low-temperature solar Rankine cycle, typical solar flat plate collectors or evacuated

tube collectors could be used instead of an evaporator [9]. The selection of an appropriate

organic working fluid to be used in the low-temperature solar organic Rankine cycle is one of

the primary things to be considered. A number of recent publications have examined a variety

of refrigerants and hydrocarbon candidates as potential working fluids that would be

appropriate for low-temperature Rankine cycles [1014]. The present study investigates the

recuperative solar Rankine cycle that uses R245fa, a dry working fluid that has favorable

availability,durability and stability, and safety characteristics. Results were obtained from an

experimental prototype constructed at Tianjin University for typical winter weather

conditions in Tianjin (39_06N 117_10E).


3/15

3 | P a g e

2.WORKING FLUID SELECTION

The selection of an appropriate working fluid plays an important,critical role for ORC

systems. Practically, the working fluid for a low-temperature Rankine cycle system should

exhibit characteristics such as being nonflammable, non-toxic, and low saturation pressures,

such that very high pressure equipment could be saved. In this study, R245fa has been

selected as a working fluid because it possesses the characteristics mentioned above, and it

also has a high molecular weight, which could reduce the rotational speed or the turbine

stages, allowing for reasonable mass flow rates and turbine nozzle areas [20], meanwhile,

R245fa is also a favorable working fluid environmentally. Relevant properties of R245fa are

shown in Table 1.

Fluid Molecular

weight(g/mol)

Normal

boiling

point(c)

Critical

pressure(mpa)

Critical

temperature(

GWP

R245fa 134.05 15.14 3.65 154.04 950

One of the primary characteristics that differentiate various working fluids is the slope of the

fluids saturated vapor curve on a temperatureentropy (TS) diagram. Depending on

whether the fluids saturated vapor curves slope is positive, negative, or infinite; the fluid

can be classified as a dry, wet, or isentropic fluid, respectively [14]. Since

superheating is typically not feasible for low-temperature Rankine cycles, this characteristic

is particularly important as it determines if expansion from a saturated vapour state results in

a state inside the vapor dome. Expansion into a state inside the vapor dome is undesirable and

should be avoided, because liquid droplets that form during expansion can erode and heavily

damage the turbine blades. For a dry working vapour leaving the turbine is often significantly

superheated still at ahigh temperature. By introducing a recuperator that utilizes this sensible

heat to preheat the fluid prior to entering the collector,less waste heat is rejected to the

condenser which potentially could, improve the cycle efficiency significantly.


4/15

4 | P a g e

3.EXPERIMENTAL METHODS

The present experimental prototype, shown in Fig. 1, primarily consists of a flat plate solar

collector, a throttling valve, an air cooled condenser, a liquid tank, a feed pump, a radiometer

and a data acquisition system. The parameters of primary experimental components used are

listed in Table 2. It should be noted that in the present study, the process of preheating the

liquid fluid is simulated by using an electric tracing ribbon, where the heat added can be

controlled simply by adjusting the voltage of the ribbon. It was verified that the air cooled

condenser possessed a large enough heat transfer area such that the working fluid vapor could

be completely condensed. For the experiment flat plate collectors with internal tube diameters

of 12 mm were used. The collector aperture area was 0.6 m2 and was composed of a glass

cover sheet and a high efficiency solar absorber plate with a 0.95 absorptivity and 0.17

emissivity. During the experiment, the flow rate and the pressure in the collector can be

adjusted by the flow valve and the feed pump, respectively. These devices also allows for the

saturation temperature at which R245fa boils at to be controlled during the experiment as

well. Due to the relatively small size of the present experimental prototype, a suitable

expander is not readily available on the market; therefore, a throttling valve is used to

simulate the expansion process. Although the thermodynamic process of flow through a

throttling valve and a true expander are quite different, it nonetheless provides a starting point

and a method for which to conduct the present study. Since a true expander is not installed in

the experimental prototype; no electricity is actually produced however, the cycles efficiency

can be estimated with a thermodynamic analysis. In analysis, an expander isentropic

efficiency is set to 0.85.


5/15

5 | P a g e


6/15

6 | P a g e

4. Thermodynamic analysis

For the thermodynamic analysis, state properties for R245fa (also shown in Fig. 1) were

determined using REFPROP 8.0 [22], a computer program developed by the National

Institute of Standards and Technology (NIST) that uses modern, highly accurate equations of

states. In the analysis, the following assumptions often used in power cycle analysis, were

also made:

1. Cycle components are considered as steady-state-flow devices.

2. Changes in fluid kinetic and potential energy are assumed to be small and negligible.

3. An expander isentropic efficiency gT of 0.85 is assumed.

4. Frictional losses due to pipe walls are assumed to be small; therefore, pressure drops in the

heat exchangers and piping have been neglected.

Based on the aforementioned assumptions, a thermodynamic analysis is carried out to

determine several important metrics uponto measure the ORC effectiveness. These metrics,

defined in Eqs. (1)(6), include the power output from the expander, the parasitic power

consumed by the pump, the heat collection rate of the collector, the Rankine cycle, or

thermal, efficiency, the collector efficiency, and the overall system efficiency.

where h1, h0 2, h0 4, h5 and h6 are the enthalpies of corresponding state points in the

experimental prototype shown in Figs. 1 and 2, m_ is the fluid mass flow rate that can be

determined from the experiments measured volumetric flow rate, I is the solar radiative flux

incident on the collectors inclined surface, and A is the collectoraperture area.


7/15

7 | P a g e

5. Performance analysis and results discussion

The presented experimental results in Fig. 3 and Fig. 4 were conducted on December

17th and December 25th 2008, respectively. For the first set of results in Fig. 3, the

volumetric flow rate during the experiment is held constant at 1.3 l/h. For the second set ofresults, the volumetric flow rate is variable and manually adjusted between values of 0.975

l/h and 2.275 l/h during the experiment. The solar radiative flux is shown in Figs. 3a and 4a

for the 17th and 25th of December 2008, respectively.

In order to evaluate the cycle performance, the experiments were conducted near noon

local time, when the solar radiations variation is small; the average values for the incident

solar radiative flux were 890 W/m2 and 931 W/m2, respectively. Figs. 3b and 4b show the

pump inlet and outlet temperatures and the ambient temperature. It can be seen that the

temperature difference between the pump inlet and outlet are about 1.5 _C and 0.7 _C,

respectively. Due to the temperature testing point exposures in the atmosphere, the ambient

temperature fluctuates obviously and is lower than the pump inlet temperature. Based on the

measured temperature and pressure, it was determined from REFPROP that the working fluid

at the pump inlet is subcooled, allowing for stable operation of the pump, without concerns

over cavitation. In Figs. 3c and 4c, the collector inlet and outlet temperatures are shown. In

Fig. 3c, for the condition of constant flow rate, the collector outlet temperature increases with

increasing collector inlet temperatures, namely caused by greater heat addition in the

recuperator. However, the rate at which the outlet temperature increases is not the same as

that of the inlet temperature. This suggests that the heat absorbed by the collector is

decreasing with increasing inlet temperature. This result is due to the fact that with the fluid

coming into the collector at a higher temperature causes the fluid to vaporize sooner in the

collector. This results in a larger fraction of the collector piping to be filled with vapor. Vapor

has a lower thermal conductivity and is a poorer heat transfer fluid when compared to liquid;

therefore, the collector is unable to transfer as much heat to the working fluid resulting in

high collector temperatures. The collector then dissipates the additional heat to the ambient,

which causes the collector efficiency, as expected, to decrease, as more heat is lost to the

ambient instead of absorbed by the working fluid. In an effort to reduce this negative effect

on system performance, eight different volumetric flow rates are examined during the

experiment ranging from 0.975 l/h to 2.275 l/h. It can be seen in Fig. 4c, for the variable flow

rate case, the collector outlet temperature decreases from 77.21 _C (11:47) to 70.20 _C

(12:58) even though the inlet temperature is increasing. This indicates that the flow rate of


8/15

8 | P a g e

R245fa is one of the key factors that determines collector outlet temperature and efficiency;

which will be shown in the following sections.

Figs. 3d and 4d show the measured pressures at the outlets of the collector and throttling

valves. For Fig. 4d, the volumetric flow rate for R245fa is also shown; the flow rate is held

constant at 1.3 l/ h in Fig. 3d and therefore has been omitted from the figure. For Fig. 3d, due

to the constant flow rate, the collector outlet pressure is relatively constant at 0.274 MPa and

the fluid receives enough heat while in the collector to become superheated. The valve outlet

pressure is dependent on the ambient temperature, since the condenser is air-cooled and

requires that the working fluid to have a saturation temperature that is sufficiently greater

than the ambient such that it will be able to condense completely.


9/15

9 | P a g e


10/15

10 | P a g e

The condensing pressure imposes a steady state pressure difference of about 0.147 MPa

across the expander; due to this small pressure difference, the resulting calculated power

output is also small. The pressure difference can however, be increased by manually

regulating the volumetric flow rate, as shown in Fig. 4d. With the flow rate changing from

0.97 l/h to 2.275 l/h, the collector outlet pressure correspondingly increases from 0.227 MPa

to 0.350 MPa. The maximum observed pressure difference is 0.211 MPa. By increasing the

collector pressure, the saturation temperature of the fluid increases and correspondingly the

average heat addition temperature; from Carnot considerations, this would result in a higher

first law efficiency.

The heat absorption rate in the collector and the power output from the expander are

presented in Fig. 5a and Fig. 6a. Fig. 5 shows that for the constant flow rate condition, an

overall trend of decreasing heat absorption rate is observed; which correspondingly lowers

the collector efficiency. The heat absorption rate is related to the increased collector inlet

temperature, which has already been discussed. For the variable flow rate case, the heat

absorption rate curve of Fig. 6a mirrors the volumetric flow rate curve of Fig. 4d. As

mentioned previously, the flow rate of R245fa is a key factor that affects the collector

efficiency when there is sufficient solar radiation to completely vaporize the fluid in the

collector. In the case of constant flow rate, by introducing a recuperator, the amount of heat

absorption required in the evaporator decreases, and if a solar fluid is used as an intermediate

system, a recuperator would reduce the required flow rate of solar heat carrier fluid.

However, for direct vaporization system, where the power cycles working fluid directly

absorbs the solar radiation, the introduction of a recuperator leads to decreased collector

efficiencies.

The power output of the system in a large part is dictated by the pressure difference

across the expander, or in the present experimental system, the throttling valve. For the case

of constant flow rate on December 17th, the power output was calculated to vary between

5.23W and 5.47 W; this small power output can be attributed mainly to the small pressure

difference across the throttling valve. When the pressure difference across the expander is

increased, the calculated power output reaches 13.32W at a flowrate of 2.275 l/h. Using Eqs.

(4) and (5), the transient collector and Rankine cycle efficiency are calculated and shown in

Figs. 5b

and 6b. For the constant flow rate case, the calculated collector efficiency decreases from

22.9% to 22.1% due in part to the fluid possessing a large degree of superheat at the collector

outlet, reducing the average heat absorption temperature and increasing the collectors heat


11/15

11 | P a g e

loss to the ambient. For the variable flow rate case, the calculated collector efficiency shows

several step-wise increases, with a maximum value of about 37.60%.

The calculated values for the Rankine cycle efficiency for the constant flow rate case was

about 3.67%. From Fig. 5b for the variable flow rate case, it can be seen that the Rankine

cycle efficiency reaches a maximum of 6.43% at a flow rate of 2.275 l/h, indicating that the

Rankine cycle efficiency could be significantly improved by manually regulating the flow

rate. It should be noted that, although increasing the working fluid flow rate also increases the

pump power consumption; this additional parasitic power is minor compared to that of the

power output with the net effect being that the cycle efficiency increases as a whole.


12/15

12 | P a g e

The overall efficiencies are determined from Eq. (6), the calculated values are presented in

Figs. 7 and 8, for the constant and variable flow rate cases, respectively. It can be seen that

for the constant flow rate case, the overall efficiency is quite low; the overall systemefficiency decreases slightly from 0.84% to 0.81% due to a decrease in collector efficiency.


13/15

13 | P a g e

For the variable flow rate case though, the overall efficiency reaches a maximum of 2.42%

for a flow rate of 2.275 l/h, which is about three times the system efficiency obtained for the

constant flow rate case.

6. Summary and conclusion

An experimental investigation on low temperature solar recuperative Rankine cycle

has been performed by using R245fa, a dry working fluid. Experiments were conducted

with two different working conditions: constant flow rate and variable flow rate. The main

results are summarized as follows:

1. Counter to what was expected, for the constant flow rate case, the Rankine cycle efficiency

did not improve by introducing a recuperator. Instead, the recuperator caused the fluid to be

superheated to a large degree upon exiting the collector, raising the temperature of the

collector and decreasing its efficiency.

2. By combining recuperation and manual flow rate regulation, system performance was

observed to improve significantly. By regulating the flow rate, the degree of superheating in

the vapour prior to expansion could be smaller, resulting in higher average heat addition

temperatures. From Carnot considerations, this results in greater power output, while also


14/15

14 | P a g e

enhancing collector efficiency by reducing the fraction of the collector occupied by vapor.

Results showed the overall efficiency of this experimental prototype reaching 2.42% for

variable flow rate compared to 0.84% for constant flow rate.

3. Since the flow rate was observed to greatly influence the collector efficiency and Rankine

cycle efficiency, a further study on the optimization of flow rate regulation is needed, such

that the fluid could be vaporized only to a saturated vapor state and unnecessarily superheated

could be avoided.

4. In the present study, the Rankine cycle and overall efficiency are low due to low collector

outlet temperatures. In subsequent studies, evacuated tube solar collectors could be used

instead of flat plate ones to increase the maximum temperature of the cycle and improve the

cycle performance. At the same time, insulating the components of the ORC could also be

taken to reduce the amount of heat loss in the system.


15/15

15 | P a g e

7.References

J.L. Wang, L. Zhao , X.D. Wang Department of Thermal Energy and RefrigerationEngineering, School of Mechanical Engineering, Tianjin University, No. 92, Weijin Road,

Tianjin 300072, PR ChinaReceived in revised form 12 December 2011 Accepted 7 January

2012[1] Xu F, Goswami DY. Thermodynamic properties of ammoniawater mixtures for power-

cycle applications. Energy 1999;24:52536.

[2] Xu F, Goswami DY, Bhagwat Sunil S. A combined power/cooling cycle. Energy

2000;25:23346.

[3] Manolakosa D, Papadakisa G, Mohameda Essam Sh, Kyritsisa S, Bouzianasb K. Design

of an autonomous low-temperature solar Rankine cycle system for reverse osmosis

desalination. Desalination 2005;183:7380.

[4] Manolakosa D, Papadakisa G, Kyritsisa S, Bouzianasb K. Experimental evaluation of an

autonomous low-temperature solar Rankine cycle system for reverse osmosis desalination.

Desalination 2007;203:36674.

[5] Zhang XR, Yamaguchi H, Uneno D. Experimental study on the performance of solar

Rankine system using supercritical CO2. Renew Energy 2007;32:261728.

abstract an experimental study is conducted to investigate the performance of a low

Documents