Performance of hybrid refrigeration system using ammonia

Applied Thermal Engineering 62 (2014) 560e565

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Performance of hybrid refrigeration system using ammonia G. Lychnos*, Z. Tamainot-Telto School of Engineering, University of Warwick, Coventry CV4 7AL, UK

h i g h l i g h t s
A mechanical compressor was combined with up to 5 pair bed adsorption generator.
The hybrid’s refrigerating capacity ranges between 4 kW and 24 kW.
The driving temperature varies from 100  C to 250  C.
The driving temperature rises significantly at small number of adsorbent beds.
The system preformed best at low driving temperatures with a 5 pair adsorbent bed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 May 2013 Accepted 12 October 2013 Available online 21 October 2013

This paper investigates the performance of a hybrid refrigeration system that combines sorptioneconventional vapour compression refrigeration machine driven by dual source (heat and/or electricity). The dual source makes the system highly flexible and energy efficient. The ammonia refrigerant (R717) is used in both adsorption and associated conventional refrigeration cycles. The model of thermal compressor corresponds to a multiple pair of compact adsorption generators operating out of phase with both heat and mass recovery for continuous cooling production and better efficiency. Each generator is based on a plate heat exchanger concept using the activated carboneammonia pair. The model of conventional vapour compressor is a reciprocating compressor from Frigopol. The hybrid refrigeration performances are presented mainly for ice making and air conditioning applications (TC ¼ 40  C, 5  C < TE < 20  C). The exhaust temperature of the compressor (driving temperature for thermal compressor) varies from 90  C to 250  C. The results show a cooling production ranging from 4 kW to 12 kW with back-up mode (both cycles not operating simultaneously) and from 8 kW to 24 kW with complementary mode (both cycles operating simultaneously). The effective overall COP based on the total equivalent heat rate input varies from 0.24 to 0.76. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Adsorption Vapour compression Ammonia Hybrid refrigeration COP

1. Introduction According to the Climate Change Act 2008 the UK has to cut greenhouse gas emissions by 80 per cent below 1990 levels by 2050 and 34 per cent by 2020 [1]. To achieve a 34 per cent decrease by 2020 would require the reduction of 2010 total emissions (590.4 MtCO2e) by 84.6 MtCO2e. Carbon dioxide emissions accounted for 84 per cent of total UK greenhouse gas emissions in 2010, attributed to the energy supply sector (39%), road transport (22%), residential sector (17%) and business (15%). The total final consumption of energy was 159.1 MtCO2e in 2010 with the domestic (30.5%), transport (35%) and industrial (17.3%) sectors being the main consumers. Electricity accounted for 17.7% of the total energy consumption by final users and 21 per cent of the domestic energy * Corresponding author. Tel.: þ44 2476 573 015; fax: þ44 2476 418 922. E-mail addresses: [email protected], [email protected] (G. Lychnos). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.10.013

consumption. Most of the electricity in the domestic sector is used for powering household appliances. In 2009 about 17% was used by cold appliances and approximately 14% for space heating. It is the main fuel used for air-conditioning of human dwellings and refrigeration. Improvements in the efficiency of heating and cold appliances had a direct effect on energy consumption the last twenty years. The efficiency of cold appliances improved significantly since 1990 and by 2010 they consumed less electricity by an average of 52% in the UK. In fact since 1990 and by 2010 the electricity consumption by cold appliances in the domestic sector has reduced by 7.4%. Thus the improvement of the energy efficiency of cold appliances is still of primary importance in reducing their carbon footprint. Apart from using electricity and therefore emitting indirectly carbon dioxide, the conventional refrigerating and heat pump systems (mechanical vapour compression technology) also use

G. Lychnos, Z. Tamainot-Telto / Applied Thermal Engineering 62 (2014) 560e565

Nomenclature A cp COP D h H L m M n N P q Q R t T U v _ w W x

heat transfer area (m2) specific heat (J kg1 K1) coefficient of performance cylinder diameter (m) specific enthalpy (J kg1) specific heat of sorption (J kg1) length of stroke (m) mass flow rate (kg s1) mass (kg) number of cylinders rotation speed (rad s1) pressure (Pa) volumetric flow rate (m3 s1) capacity (W) gas constant (J kg1 K1) time (s) temperature (K) heat transfer coefficient (W m2 K1) specific volume (m3 kg1) specific work (J kg1) mechanical power (W) concentration (kg kg1)

Greek symbols efficiency (%)

h

refrigerants that can be harmful on the environment causing depletion of the ozone layer and global warming phenomena. Sorption systems can be an alternative to the vapour compression ones because they are heat driven (using waste heat or solar energy) and utilise natural refrigerants (ammonia, water, alcohols) that have low or zero global warming potential (GWP) and zero ozone depletion potential (ODP). However, sorption systems have some undesirable characteristics; they are usually big sized with high volume for given cooling capacity and have low COP. Recent advances though in adsorption technology have made possible producing more compact and lighter systems with higher efficiency [2,3]. Hybrid systems comprise sorption and vapour compression technology, putting together the advantages of both technologies and thus giving more flexibility in operation, tolerance under extreme temperature conditions and reduced cost. Research on hybrid systems that combine mechanical and thermal compressors has been focused on the concept of operating them in series [4,5]. Here we put forward the concept of parallel operation. In this work the performance of a hybrid refrigeration system utilising ammonia (R717) is investigated. In particular, simulations of a thermal compressor (adsorption generator developed at Warwick University) and a conventional mechanical compressor (Frigopol [6], separating hood compressor 7-DLZC-1.5) are presented. The simulations were carried out at 40  C condensing temperature and with evaporating temperatures ranging from 5 to 20  C. Based on the results we identify how these two processes can be matched efficiently for operating under different conditions and for different applications (i.e. air conditioning and ice production).

g a

561

ratio of specific heat slope

Subscripts a adsorbate ads adsorption C condensing c carbon e refrigerating E evaporating f fluid h heat in inlet isen isentropic LM log mean mc mechanical compression p pressure Sys system sat saturated v volume LM log mean out outlet p pressure pp power plant pt power line transmission w wall 1e4 states in mech. vapour compression cycle

to produce continuous cooling while heat and mass recovery takes place to improve the efficiency. The sorption generator is of plate heat exchanger type developed by Critoph and Metcalf [7] and uses the activated carboneammonia pair. The model is a finite difference model created in MatlabÒ that has been published in literature [8,9]. However, it is worthwhile mentioning that key governing equations for the model are related to the energy balance on the wall, the thermal fluid and the generator bed:

Mw cpw

vTw ¼ ðUAÞfw TLM fw  ðUAÞwc ðTw  Tc Þ vt

for the wall (1)

vTf vt   mcpf Tf in Tf out ¼ ðUAÞfw DTLM fw for the thermal fluid

Mf cpf

(2)

Mc cpc þ xcpa  Mc H


vTc vt

vx ¼ ðUAÞwc ðTw  Tc Þ vt

for the generator bed

(3)

Strictly cp should be cv, however the distinction is unimportant for a liquid or solid for which cp z cv (cp w 4734 J kg1 K1). H is the heat of sorption and is given by the following expression:

Tc Tsat

2. Model elaboration

H ¼ Ra

The adsorption model can simulate the performance of a thermal compressor which consists of multiple pairs of compact adsorption generators. The generators operate out of phase in order

where: R is the gas constant at the bed pressure P and temperature Tc (R w 488 J kg1 K1); a is the slope of the saturated ammonia line on the Clapeyron diagram (a ¼ 2823.4 K); Tc is the carbon

(4)

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temperature (K); Tsat is the saturation temperature corresponding to the gas pressure (K). For the modelling of the mechanical compressor is based on the steady state thermodynamic method. The separating hood reciprocating compressor 7-DLZC-1.5 [6] was chosen because it is compatible with pure ammonia and ammonia mixtures. Based on technical data generated from Frigopol’s software, we have developed a model in MatlabÒ that can simulate the performance of the compressor under various operating conditions. Both isentropic and overall volumetric efficiencies are included in the model for realistic operating conditions. The compression process is considered isentropic and the gas ideal therefore:

ammonia at the outlet inlet of the evaporator (J kg1) and mf the mass flow rate (kg s1). The coefficient of performance of the vapour compression refrigerating machine is then:

Pvg ¼ Constant

where: PE and PC are evaporating and condensing pressures respectively (Pa); g is the ratio of specific heat (g ¼ 1.3 for ammonia gas); T1 is the gas suction temperature (K). The effective discharge temperature T2 is calculated implicitly from the current specific enthalpy h2 at the outlet of the compressor:

(5)

where: P and v are the gas pressure (Pa) and specific volume (m3 kg1) respectively; g is the ratio of specific heat (g ¼ 1.3 for ammonia gas). Thus the specific work of isentropic compression in J kg1 is given by the following:

 _ ¼ w

(6)

where: PE and PC are evaporating and condensing pressures respectively (Pa). v is the specific volume (m3 kg1). The theoretical flow rate generated (m3 s1) by the piston displacement of the reciprocating compressor is given by the following expression:

q ¼

pD2 LNn N 4

$

(7)

60

where: D is the cylinder diameter (m), L is the length of stroke (m), N is the rotation speed (RPM) and n is the number of cylinders [10]. By taking into account the compressor volumetric efficiency hv, the effective mass flow rate of ammonia gas (kg s1) is:

mf ¼

q hv v

(8)

where: q is the theoretical flow rate (m3 s1); hv is the compressor volumetric efficiency; v is the specific volume (m3 kg1) taken at the inlet of compressor operating conditions. The volumetric efficiency is given as a function of the pressure ratio based on the characteristic line of the chosen compressor:

hv ¼ 0:955



PC PE

0:206 (9)

The actual mechanical power of compression is defined as:

W ¼

_ mf w

hisen

(10)

_ is specific work where: mf is the effective mass flow rate (kg s1); w (J kg1) and hisen is the isentropic efficiency given as a function of the suction temperature T1 (K):

hisen ¼ 0:6039 þ 0:00207ðT1  273Þ

(11)

The refrigerating capacity Qe (W) is defined as:

Qe ¼ mf ðh1  h4 Þ

Qe W

(13)

The isentropic discharge temperature Tisen was calculated as:

 Tisen ¼ T1

PC PE

_ h2 ¼ h1 þ w



 g1  PC g PE v 1 g1 PE

g

COPr ¼

(12)

where: h1 is the specific enthalpy of ammonia outlet of the evaporator with 10 K of superheat (J kg1); h4 is the specific enthalpy of

g1 g (14)

(15)

_ are specific enthalpy of ammonia outlet of the where: h1 and w evaporator with 10 K of superheat (J kg1) and specific work (J kg1). An example of illustration of the theoretical concept of a hybrid system is presented in Fig. 1. The compressors can either operate at the same time with common loop or only one at one time. Another possible setup could be that of both compressors operating at the same time on two separate loops. Detailed descriptions of numerous layouts of hybrid systems along with key components are well documented [11]. The operating conditions of the simulations carried out with the adsorption model were set to be the same to the mechanical compressor model’s simulations. Hence by comparing the mass flows of the refrigerant in both cycles we were able to predict the number of the regenerative adsorption beds required to equally match each other’s refrigerating capacity within less than 5% relative error. It is worth noting that the activated carbon type used in the simulations was the SRD1352/2 manufactured by Chemviron carbon Ltd. The adsorbent characteristics of this carbon type (compacted sample) can be found in Tamainot-Telto et al.’s work [12]: the thermal compressor prototype that is made of a pair of 1 kg activated carbon loaded in high thermal performance reactor has a typical maximum cooling capacity of about 1.6 kW with low grade driven temperature as 90  C (Te w 10  C and Tc w 40  C). Futhermore the thermal compressor prototype includes both mass and heat recoveries. The thermal compressors are considered operating in parallel in the modelling: this is not necessary an optimum way of operating multiple thermal compressors [13] but could be cost effective in manufacturing. In order to compare the COPs of the two systems (mechanical compression with thermal compression) we converted the COP of the mechanical compressor which runs on electricity, into a more realistic one that takes into account the efficiency of the thermal power plant (conventional) and also the transmission and distribution (T&D) losses. Thermal power plants convert thermal energy to electricity. A typical value of the electricity production efficiency for conventional coal power plants is 35% [14]. Losses occur on the way from the power plant to the end user due to transmission and distribution that can be as high as 21.1% in India [15]. In our calculations we have assumed 35% and the losses 12% (corresponding to 88% of power line transmission efficiency). Therefore the intrinsic coefficient of performance is defined as:

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Fig. 1. Layout of a hybrid system where only one compressor operates at one time on common refrigerant loop via a 3-way valve [11]. Table 1 The simulation results of the mechanical compressor. Qe

Wreal

Qc

COPr

Tc

Te

mref

Tdis,isen

Pc/Pe

Tdis,real

COPr*

4.26 5.31 6.57 8.07 9.84 11.94

1.43 1.50 1.53 1.53 1.46 1.33

5.68 6.81 8.10 9.59 11.31 13.27

2.99 3.54 4.28 5.28 6.72 8.98

40 40 40 40 40 40

5 0 5 10 15 20

0.00389 0.00483 0.00594 0.00727 0.00883 0.01067

117.6 107.8 98.6 90.2 82.3 74.9

4.36 3.62 3.02 2.53 2.14 1.82

169.3 150.1 132.2 116.2 101.9 88.7

0.93 1.11 1.33 1.64 2.05 2.76

Table 2 The predicted driving temperatures and mass flow rates of ammonia for 1e5 pair adsorbent bed systems. Te ( C)

5 0 5 10 15 20

Mass flow rate mref (kg/s)

Driving temperature

1 Pair

2 Pairs

3 Pairs

4 Pairs

5 Pairs

1 Pair

2 Pairs

3 Pairs

4 Pairs

5 Pairs

na na na na na na

0.00390 0.00491 na na na na

0.00404 0.00495 0.00607 0.00731 0.00876 0.01060

0.00379 0.00512 0.00611 0.00741 0.00871 0.01074

0.00375 0.00507 0.00589 0.00764 0.00934 0.01066

>250 >250 >250 >250 >250 >250

140 188 >250 >250 >250 >250

120 120 122 155 185 250

110 110 108 108 125 150

106 104 100 100 100 115

COPr ¼ hpp hpt COPr

(16)

where: hpp and hpt are power plant efficiency (35%) and power line transmission efficiency (88%) respectively. To estimate the performance of the hybrid system when both compressors operate at the same time, we calculate the overall coefficient of performance based on the total equivalent heat rate inputs:

COPSys ¼

2 1 þ COP *

1 COPr;ads

(18)

r

(17)

Table 3 The predicted refrigerating capacities and coefficients of performance COPr for 1e5 pair adsorbent bed systems.

5 0 5 10 15 20

COPSys ¼

3. Simulation results and discussion

Qe;ads þ Qe;mc Qh;ads þ Qh;mc

where: Qe,ads and Qe,mc are cooling capacities for adsorption system and vapour compression system respectively (W); Qh,ads and Qh,mc

Te ( C)

are effective heat rate inputs for adsorption system and vapour compression respectively (W). When operating at maximum refrigerating capacity, Qe,ads is equal to Qe,mc ¼ Qe, therefore equation (17) could be rearranged as follow:

Refrigerating capacity Qe (kW)

COPr

1 Pair

2 Pairs

3 Pairs

4 Pairs

5 Pairs

1 Pair

2 Pairs

3 Pairs

4 Pairs

5 Pairs

na na na na na na

4.17 5.28 na na na na

4.32 5.32 6.55 7.92 9.52 11.56

4.05 5.50 6.59 8.03 9.48 11.72

4.01 5.45 6.36 8.28 10.16 11.63

na na na na na na

0.23 0.26 na na na na

0.21 0.25 0.29 0.34 0.35 0.30

0.17 0.22 0.27 0.32 0.39 0.43

0.14 0.19 0.23 0.30 0.35 0.44

The predicted refrigerating capacity, actual work of compression, heat rejected at the condenser, COPr, mass flow rate, isentropic discharge temperature, compression ratio, actual discharge temperature and COPr* of the mechanical compression can be seen in Table 1. The driving temperature of the adsorption cycle is predicted along with the cooling capacity and mass flow of ammonia for 1, 2, 3, 4 and 5 pair adsorbent bed systems. The results of the simulations are shown in Tables 2 and 3: only 5 pair adsorbent bed system operating at low grade driving temperature (100  Ce 120  C) is able to match the vapour compression cooling capacity within a large span of evaporating temperature (5  C to 20  C). Operation of one pair system seems unfeasible because of the high driving temperatures required (>250  C) while a two pair system can only find application in ice production. In Fig. 2 the predicted mass flow rate mref and refrigerating capacity Qe are illustrated against the evaporating temperature Te for

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4. Conclusion

Fig. 2. Mass flow of ammonia mref and refrigerating capacity Qe of the mech. vapour compression system versus the evaporating temperature Te compared to an adsorption system with five (5) pairs of adsorbent beds (Tc ¼ 40  C).

a 5 pair system. Similar figures can be obtained for 3 and 4 pairs as expected because we have decided to match the refrigerant mass flow rate of both compression systems (mechanical and thermal) within 5% difference. This means that the driving temperature of the adsorption cooling system is variable from 100  C to 250  C while the exhaust temperature of the mechanical compressor ranges from 89  C to 170  C. Fig. 3 shows the simulation results of the COPr and Qe against Te for the mechanical vapour compression system only: the cooling capacity varies from 4 kW to 12 kW while the coefficient of performance (COPr) ranges from 3 to 9 and will drop with a factor of about 3 due to both thermal plant (conventional) and also the transmission and distribution (T&D) losses. The coefficients of performance for both cycles (adsorption and mechanical vapour compression) against the evaporating temperature for a five pair adsorbent bed system are also illustrated in Fig. 3: the mechanical compression system is about 5e6 time efficient than the thermal compression with the current parallel type of layout. The overall hybrid system coefficient of performance COPSys, taking into account the efficiency of the thermal plant and transmission and losses, is therefore highly affected and ranges between 0.24 and 0.76.

The performance of an ammonia hybrid refrigeration system that combines adsorption and mechanical vapour compression technologies has been investigated under typical condensing temperature (40  C) and evaporating temperatures ranging from 5  C to 20  C. The simulation results presented here regard a reciprocating vapour compressor from Frigopol when combined with up to 5 pair adsorbent bed thermal compressor. When the two systems operate together, the refrigerating capacity of the hybrid ranges between 8 kW and 24 kW approximately. When only one system operates, the refrigerating capacity of the hybrid ranges between 4 kW and 12 kW approximately. The driving temperature varies from 100  C to 250  C. Specifically for 3, 4 and 5 pairs adsorbent bed, the driving temperature ranges between 120  C and 250  C, 110  C and 150  C and 100  C and 115  C respectively. The COPr of 3 and 4 pair systems is higher than that of 5 pair system at low evaporating temperatures (5  C to þ5  C). However, the driving temperature rises significantly when utilising a system with small number of adsorbent beds. The number of beds choice depends on the temperature of the heat source available to drive the adsorption cycle. The best performance at low driving temperatures can be achieved with a 5 pair adsorbent bed for ice production, refrigeration and air conditioning applications. 5. Future work The future work will be focused on the design, building and testing a proof of concept 10 kW cooling capacity hybrid refrigeration machines using ammonia mixture R723 (40% dimethyl ether, 60% ammonia) which is compatible with conventional refrigeration copper alloy (CuNi10). Prior to this work, the evaluation of thermophysical properties of various activated carbon-R723 pairs (porosity, specific heat, permeability and thermal conductivity) and other hybrid configuration models including strategy control are to be carried out. Acknowledgements The project is supported by EPSRC (Grant EP/J000876/1) and Chemviron Carbons Ltd (Lockett Road, Lancashire WN4 8DE, UK). References

Fig. 3. Refrigerating capacity Qe, actual coefficient of performance COPr of the mechanical vapour compression system, predicted COP of the thermal compressor (5 pairs) and intrinsic COPr* of the mechanical compressor along with the overall COPSys of the system against the evaporating temperature Te.

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