Lab-scale Organic Rankine cycle for low temperature

geothermal energy source Yuyuan Xu (2014219)

Department of Electronic and Electrical Engineering

University of Glasgow, charity number SC004401

Background Due to the increasing demand of energy consumption and the

target for using renewable energy, Organic Rankine cycle

(ORC) has become the attractive option as it is suitable for

low-grade renewable energy heat source, such as geothermal

energy.

Objective The purpose of the project is to model and design a lab-scale

Organic Rankine Cycle system for utilising geothermal energy

and to anaylise how the temperature heat source affect the

efficiency of ORC systems.

Method The thermodynamic properties of the working fluid and the

ORC performance are evaluated with Excel spreadsheet

which linked to the REFPROP 9.0.

Conclusion The maximum thermal efficiency of ORC is ∆T=70℃. A larger

temperature difference result in higher thermal efficiency of

ORC η and Carnot efficiency ηcarnot. In addition, there is a

contrary relationship between second law efficiency and the

irreversibility rate in ORC cycle.

Reference Danfoss. Datasheet for Brazed heat exchanger XB. (2010)

Air Squared Mfg. Datasheet forE15H22N4.25 1KW Scroll Expander. (2012)

Heat source temperature Th(℃) 65 70 75 80 85 90

Irreversibility rate Φ (kW)

0.11580 0.12397 0.13188 0.13953 0.14695 0.15412

Second law efficiency η2nd (%)

39.5611 38.1944 36.9820 35.8990 34.9258 34.0464

For approaching energy

balance in heat changer,

Brazed heat exchanger

XB 06H-1 was chosen

as evaporator type, so

UA was determined.

UA=127.731 W/K

Figure 3.Flow chart of ORC cycle

Figure 2. T-S diagram of ORC for R245fa

Figure 4. Heat source temperature

difference and theoretical thermal

efficiency

Working fluid: 245fa

Given heat source temperature and heat sink

temperature

Constant parameters in the proposed ORC system:

ηpump, ηexp, mc, mh, Wexp

ηcarnot =TH − Tc

TH

η2nd =

W exp

W exp +Itotal

evap

pump

Q

WW

exp

Φ =Itotal

Qevap

P1=P4

P2=P3

S1=S2,is

S3=S4,is

Figure 1. Schematic diagram of ORC system under consideration

Figure 5. Heat source temperature

difference and Carnot efficiency

Table 1. Variations of irreversibility rate and second

law efficiency with increasing heat source temperature Wpump = h2 - h1

Qevap = h3 - h2

Wexp = h3 - h4

Qcond = h4- h1

Expander

(Air Squared Mfg, 2012)

Heat exchanger

(Danfoss, 2010)

7.25

7.3

7.35

7.4

7.45

7.5

45 50 55 60 65 70

The

ore

tica

l th

erm

al e

ffic

ien

cy o

f O

RC

η (

%)

Heat source temperature difference ∆T (℃)

7.6

7.8

8

8.2

8.4

8.6

8.8

9

9.2

9.4

45 50 55 60 65 70

Effi

cie

ncy

of

Car

no

t η

carn

ot (

%)

Heat source temperature difference ∆T (℃)