Scholarly article on topic 'Development of a Small Capacity Solar Cooling Absorption Plant'

Development of a Small Capacity Solar Cooling Absorption Plant Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Energy Procedia
OECD Field of science
Keywords
{absorption / "solar cooling" / minichannel.}

Abstract of research paper on Materials engineering, author of scientific article — Sorin Bolocan, Florea Chiriac, Alexandru Serban, George Dragomir, Gabriel Nastase

Abstract Air conditioning systems has known a steady grow in recent decades which has a major impact on energy demand and environment. Systems using solar energy as fuel have been developed more than 100 years ago. The paper present such an alternative, prototype system, ammonia-water absorption refrigeration system (ARS) with minichannels powered by low temperature energy, using renewable sources for cooling. A short numerical calculation is made for determine the performance of the machine. The results obtained using EES program are presented and also first measurements on prototype machine.

Academic research paper on topic "Development of a Small Capacity Solar Cooling Absorption Plant"

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Energy Procedia 74 (2015) 624 - 632

International Conference on Technologies and Materials for Renewable Energy, Environment and

Sustainability, TMREES15

Development of a small capacity solar cooling absorption plant

Sorin Bolocanb'*, Florea Chiriaca, Alexandra Serbanb , George Dragomirb,

Gabriel Nastaseb

aBuilding Services Faculty, Technical Univesity of Civil Engineering, Bucharest 021414, Romania bBuilding Services Department, Faculty of Civil Engineering, Transylvanya University of Brasov 500152, Romania

Abstract

Air conditioning systems has known a steady grow in recent decades which has a major impact on energy demand and environment. Systems using solar energy as fuel have been developed more than 100 years ago. The paper present such an alternative, prototype system, ammonia-water absorption refrigeration system (ARS) with minichannels powered by low temperature energy, using renewable sources for cooling. A short numerical calculation is made for determine the performance of the machine. The results obtained using EES program are presented and also first measurements on prototype machine. © 2015PublishedbyElsevierLtd.Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD)

Keywords: absorption; solar cooling; minichannel.

1. Introduction

Currently, space heating and cooling together with water heating are estimated to account for nearly 60% of global energy consumption in buildings. They therefore represent the largest opportunity to reduce buildings energy consumption, improve energy security and reduce CO2 emissions, particularly due to the fact that space and water heating provision in some countries is dominated by fossil fuels. Meanwhile, cooling demand is growing rapidly in countries with highly carbon-intensive electricity systems such as Association of Southeast Asian Nations, China and the United States [1]. Due to the negative impact on the environment as a result of the intensive use of fossil fuels,

* Corresponding author. Tel. +40 7339 12345: fax: +40 268 548228. E-mail address: sorin.bolocan@unitbv.ro

1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4 .0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi:10.1016/j.egypro.2015.07.796

effects felt across the planet through global warming, the scientific community has shifted research towards a clean energy source and accessible all over the world, solar energy. Solar refrigeration systems has been evolved over 100 years and become more efficient. However, they still have to be improved in order to become competitive.

In Figure 1 are reviewed and classified the most popular and advanced processes for obtaining artificial cold that can use solar renewable energy. We will elaborate more on the use of one, namely the ammonia-water absorption system.

Electric process

1 Solar radiation 1

Solar thermal collector Thermal process

CHCP Combined Heating, Cooling and Power

Liquid sorbent Absorption Solid Adso orbent rption

[ Liquid sorbent |

Solid sorbent

[ HjO +LiBr| I

NH3 + HjO

Si02+H201

I Desiccant column^

Desiccant wheel

Fig. 1. Classification ofthe main processes that use solar energy for obtaining artificial cold [2, 3, 4, 5, 6].

Nomenclature

A absorber T temperature(°C)

C condenser V evaporator

CS solar collector's w mechanical power (kW)

Co consumer mass fraction, concentration (kg/kg)

COP coefficient ofperformance efficiency

E economizer

f circulation coefficient Subscripts

G generator a absorber

h enthalpy (kJ/kg) c condenser

LS liquid separator e economizer

m mass flow (kg/s) g evaporator

P pump P pump

P. pressure (bar) 0 evaporator

Q thermal power (kW)

q o specific capacity (kJ/kg)

Many of the technologies reviewed are compared in terms of performance and initial cost based on ideal assumptions by Kim and Ferreira 2008. Artificial cooling with solar energy involves the use of solar radiation converted by thermal solar panels or using solar photovoltaic panels and different thermodynamic cycles or electric process. Absorption and adsorption are comparable in terms of performance but an adsorption chiller is more expensive because of the bigger components required for the same capacity. The biggest disadvantage for absorption refrigeration system is high initial investment at about 1000€/kW [4] of which the biggest portion is for solar collectors.

The developments in cooling systems programs show a growing interest in the application of absorption systems. The first important European project SACE (Solar Air Conditioning in Europe) with more than 60 case studies, was initiated in 2002 and was supported by the European Commission. Then followed programs from IEA (International Energy Agency) Task series (25, 30, 32, 38 and 48), IEA-SHC, Solair, Solera, SolarCombi+, Medisco, Climasol. Together, Green Chiller and Tecsol projects reached at almost 1000 absorption systems documented in 2012. Now days probably the number is higher as can be seen the trend line in Figure 2.

1000 900 800 700 600 500 400 300 200 100 0 -100 —■— Docum Estimat > ented ed /y * ' j/ s) ,y Af f y * *

2006 2007 2008 2009 2010 2011 2012

Fig.2. Market development of solar cooling small capacity machine (Green Chiller, Tecsol, IEA SHC Task 38)

Optimization of energy conversion systems becomes more important due to limitations of fossil fuels and the environmental impact during their use. Opportunities for improving solar ARS can came from cheaper solar collector, chiller COP increased, lower driving temperature to work with actual solar heating systems, use of air for dissipation of heat instead of cooling towers. Two types of absorption air-conditioning systems are widely used, LiBr-H20 and H2O-NH3. New working pairs have also been developed [7, 8, 9, 10] and their performance studied but there seems not to be a choice that supersedes the characteristics of the already mentioned common two [11].

Both LiBr and NH3 systems have their advantages and disadvantages. NH3 systems can work at low temperatures and air cooled condenser and absorber make them suitable for heat pump operation. NH3 is accessible and inexpensive not like LiBr which is more expensive. NH3 have high latent heat of vaporization but they have to work at high pressure and have a high grade of toxicity but have no environmental impact such as global warming potential and greenhouse effect. It is lighter than air and the leak of ammonia can easily detect smell at a concentration below a dangerous level. NH3 is corrosive to Cu and its alloys but not to steel. LiBr systems are limited to work over 5°C, can't be used as heat pumps. Also operating in vacuum condition and at high concentrations may crystalize if are not used substance for solubility like ZnBr2. The biggest advantage of LiBr is that it does not need rectifier like NH3 system but it must be kept leak free that can affect the capacity of the machine and can became corrosive in presence of O2.

Theoretical and experimental works have been done on different systems with single, half, double, triple effect cycles and have been extensive reported [Gosney, Herold, Henning, Kim, Grossman, Ziegler, Jakob, Chiriac, Serban, Boian, Gomri, Kaynakly, Sencan, Sosen, etc.] but for low temperature grade the single effect was find to be best suitable.

2. Solar powered single effect solar ammonia - water absorption refrigeration system prototype

An absorption chiller uses heat as driving energy, as compared to vapour compression chiller that use electricity. For a solar absorption cooling system, this heat is taken from sun energy. The absorption cooling system single effect has a coefficient of performance, COP, somewhere a 0.6 -0.8 according to literature [2-14]. The solar-powered cooling system, Figure 3a, generally comprises three main parts: the solar energy conversion equipment, the refrigeration system, and the consumer.

90 TTC)

Fig. 3. Solar cooling plant schematic a) and real b) [4]

The main components of the prototype single effect absorption cooling system, Figure 3b) are: the generator (G), the absorber (A), the condenser (C), the evaporator (V), the pump (Ps), the expansion valve (VL1), the reducing valve (VL2) and the solution heat exchanger (Ei). Because the subcooler efficiency is very low, about 5%, and the pressure drops affects more the performance we decided not to use it in this phase. The generator is a plate heat exchanger with 2.5 mm minichannels. After boiling, is resulting a biphasic solution, constituted by poor solution and ammonia vapors, which are separated in the liquid separator LS (SL), from where they go to the condenser, consisting in 2 mm minichannels, fined heat exchanger, and air cooled with a fan. Ammonia condenses in the condenser by removing the heat from the refrigerant vapor and the resulting liquid is laminated by the control valve VL2 where its pressure is reduced to the low pressure, after which the liquid enters the evaporator which is also a plate heat exchanger with 2.5 mm minichannels. Here, the liquid ammonia vaporizes, by taking the heat from the cooled water, which comes with 12°C. Vapors are then being absorbed in the absorber, by weak ammonia solution come from liquid separator. The absorber is an original construction, consisting of mini/micro channels, arranged in two vertical rows and have efficient finned outer surface, with superior distributor for the poor solution and lower collector for the strong solution; ammonia vapor injection, to be absorbed is done through the median distributor, connected to the mini/micro channels by individual connections [12]. The result is a strong ammonia solution, which is preheated in the economizer then being pumped into the generator by the solution pump. The solution pump is a pulse pump having a reciprocating motion. It discharges strong solution to the generator by means of a flexible sealing diaphragm The weak solution, poor in ammonia resulting in the generator, is separated in liquid separator, then is cooled in the economizer Ei, then

laminated by the control valve VLi and finally absorbed in absorber, where a strong ammonia-water solution is formed. With the preheated strong solution returning to the generator the cycle is completed. [12]

The absorber is the most important component of absorption machines, in general, its performance impacts directly in the size and energy supply of all absorption devices. Absorption cooling and heating cycles have different absorber design requirements: in absorption cooling systems, the absorber works near to ambient temperature, therefore, the mass transfer is the most important phenomenon in order to reduce the generator size and power of pumps; in the other hand, in heating absorption systems, it is important to recover the heat delivery by the exothermic reactions produced in the absorber, for this reason, the absorber heat transfer coefficient is an important parameter.[13]

3. Energy analysis

A mathematical model is developed to analyze the performance of the experimental system ARS based on the mass and energy balances for the first law of thermodynamic [6,14]. Temperatures and pressures of working fluid are based on designed values. Water and ammonia properties are obtained from standard properties of pure substances table in the ASHRAE [15], Refrigerating Plants [16] and Duhring plot. Figure 4 a) represent the schematic components and b) thermodynamic cycle in p-T-^ diagram for better state point identification in EES simulation.

Cooling water Hot water

Fig. 4. EES schematic a). Thermodynamic cycle of the installation processes in p-T-^ diagram b)

The theoretical model use input data, main assumptions and operating conditions that are presented below. The input data (operating conditions) are:

- The cooling power of the evaporator, which is fixed in all calculations: QQo ~

- The inlet and outlet temperatures of the external fluids, air, water:

• hot water = 90°C, cold water = 7-12°C, air = 35°C.

- The minimum temperature difference in the heat and mass exchangers = 3-5°C

- Concentration of vapors leaving G-SL = 0.998. The output data are:

- The pressures, temperatures, mass flows, concentrations, enthalpies, of each state point.

- The thermal or, in the case of the solution pump, mechanical power of the main components.

- The COP. Assumptions:

• Itisa steady state cycle.

• There are no heat and pressure losses.

• The refrigerant leaving the condenser is saturated liquid.

The refrigerant leaving the evaporator is evaporated completely as saturated vapor. The strong solution leaves the absorber at the absorbent temperature as saturated liquid. The solution and refrigerant valves are adiabatic. Pump is isentropic.

First the pressures are determined:

po=i(To)=fA (1)

pc=f(Tc)=pG (2)

Then we must check the degassing breath which is the difference between the strong and weak concentrations. This must be bigger than 5% otherwise circulation factor/will be larger than 13 which means a bigger mass flow of solution and a smaller COP.

=£з = f ( Pg Tg = T3) çsb =£7 = f ( Pa ta = ti)

Circulation coefficient f, evaporator specific capacity and mass flow of refrigerant are stated below:

/ = fe _ )/feb _ ) (5)

ko = h6 - h5[kJ / kg] (6)

m0 = Q Jq0 \kg Is] (7)

Mass balance is:

mss = msb - m0[kg / (8) A thermal balance exists:

Qg + Qo + Wp = Qa + Qc (9)

Where generator capacity is:

Qg = [h2 + /-1)• h3 -/■ h]■ m0 [kW] (io)

Absorber capacity is:

Qa = [h6 + (/-1) • h10 - / ■ h7] • m0[kw] (11)

Condenser capacity is:

Qc = [h2 + {/-1)• h -/• h,]• m, [kW] (12)

Economizer capacity is:

Qc = h + (/ -1) • h - / ■ hi] • m0[kw ] (13)

And finally the solution pump:

Wp = [f ■ (hs -h7] • m [kW]

Efficiency of ARS is described in the term of coefficient of performance COP which is desired output (Qo) divided by required input (Qg + Wp).

cop = — q +w

To be more precise same equations were used in EES which contains procedures for thermodynamic properties of ammonia-water solution.

In Table 1 characteristics of every state point are presented and in Table 2 heat transfer of components and performance parameters of the system. Also the properties for all states points, COP for the ammonia-water ARS are obtained using the Engineering Equation Solver (EES) [17], Table 3 and 4.

Table 1. Thermodynamic properties ofthe absorption system.

CALCULATION

State point p [bar] [°C] [kJ/kg] [kg/kg]

1 13.5 68 90 0.53

2 13.5 80 1410 0.998

3 13.5 85 150 0.44

4 13.5 35 95 0.998

5 4.97 4 95 0.998

6 4.97 7 1200 0.998

7 4.97 35 -90 0.53

8 13.5 36 -85 0.53

9 13.5 46 -58.65 0.44

10 4.97 43 -58.65 0.44

Table 2. Heat transfer of components and performance parameters.

CALCULATION

transfer

Component rate(kW)

Evaporator 5

Absorber 6.57

Generator 7.38

Condenser 5.95

Solution pump 0.14

Solution heat exchanger 4.91

Performance parameters ofARS

Circulation ratio 6.20

Coefficient of performance 0.66

Table 3.Thermodynamic properties ofthe absorption system. Table 4. Heat transfer ofcomponents and performance parameters.

EES EES

State point p [bar] T [°C] h [kJ/kg] x [kg/kg] Component Evaporator Absorber Heat transfer rate(kW) 5

1 13.51 71.1 84.39 0.53 7.08

2 13.51 80 1433 0.99 Generator 8.19

3 13.51 85 146.3 0.44 Condenser 6.26

4 13.51 35 158.9 0.99 Solution pump 0.15

5 4,97 4.4 158.9 0.99 Solution heat exchanger 4.80

6 4,97 7.4 1176 0.99

7 4,97 35 -81.39 0.53 Performance parameters of ARS

8 9 10 13.51 13.51 4,97 36 42.5 42.7 -76.18 -45.66 -45.66 0.53 0.44 0.44 Circulation ratio Coefficient of performance 6.11 0.59

4. Solar energy conversion system

As stated before one important component of solar power cooling system are solar collectors. Linked directly or through a heat exchanger they supply the energy to the generator of the machine. Each type of solar cooling plant relies on a thermal energy supplied at a certain optimum temperature. Absorption NH3/H2O driving temperature is around 95°C and for LiBr around 72°C.[1] Recent studies and experiments tried to use low grade temperature collectors so the return investment time to be smaller because efficient solar thermal collectors like evacuated tube collector or parabolic trough concentrating collector are more expensive. Figure 5 a) present the area needed for a 5 kW ARS in Brasov, Romania, for different collector type like: flat plate (FP), evacuated tube (ET) and parabolic

trough (Pt) versus maximum temperature provided by those. Also figure 6 b) shows the total global radiation measured on horizontal surface with a tilt angle of 45 degrees facing S-W during the months ofMay to August (1-4).

Sqm and Temp*10 C°

30 25 20 15 10

Sqm I Temp

• 2006

^—2007 -

Fig. 5. a) Square meter of collector versus temperature supplied by flat plate, evacuated tube and parabolic trough collector and b) global radiation measured from May to August in Brasov, Romania, S-W and 45° tilt

5. Measurements

A data acquisition system and a set oftransducers have been used to monitor the operation ofthis prototype plant. Errors between 10 to 20% have been noticed. Figure 6 shows the temperatures recorded for hot water in and out of generator, cold water from an to consumer and NH3 solution in and out of evaporator. The first result are not satisfactory but new adjustment will be made until we get closer to the calculations. In order to improve the system design of the solar powered absorption air-conditioning system, a parametric study must be carried out to investigate the influence of key parameters on the overall system performance. Experiments will be used to perform the parametric study, effects of one key parameter on the overall system performance will be monitored. Generator inlet temperature of the chiller is still the most important parameter in the design and fabrication of a solar powered air-conditioning system. Medium temperature indoor ofthe laboratory research during recorded values presented was around 20 °C.

101.00

91.00 81.00 u 71.00

|5 61.00

¡5 51.00 p

J 41.00 31.00 21.00 11.00 1.00

■ NH3 i

■T Absorber o

■Thw o

• NH3o

36925814703692581 89023467801235679

444444445

Recorded step

Fig. 6. ARS temperatures measured for hot water in and out of generator, temperature out of the absorber and NH3 solution in and out of

evaporator (i-in;o- out)

Conclusion

Prototype system operated successfully regarding the stability and temperature obtained at generator and evaporator despite the difference from calculation results. Best COP obtained was 0.43 given the actual state of the machine which has no isolation and no volume control for solution pump. Nevertheless we will continue to improve the components, absorber and liquid separator, the layout and the reduction of dimensions until we be near the predicted values.

There are many benefits of ARS:

• Can be driven by low grade thermal energy, as waste heat, biomass, solar thermal energy or heat from cogeneration so they will be a better solution for cooling in the future because of the rising prices announces for fossil fuels and the return payback will be shorter along with mass production.

• Absorption chillers are silent and vibration free.

• High protection of the environment is done due to the energy used and the refrigerants, NH3-H2O, which have zero ODP and GWP thus protecting the ozone layer and does not contribute to global warming as chlorofluorocarbons.

Owing to the fact that there is always enough roof area to install solar collectors, solar-powered integrated energy systems are capable of supplying cooling, heating, and hot water supply. For the purpose of all-weather operation, it is necessary to install auxiliary heat sources to supplement solar-powered cooling/heating systems.

References

[1] International Energy Agency. Transition to sustainable buildings, Strategies and Opportunities to 2050, Executive Summary, at http://www.iea.org/Textbase/npsum/building2013SUM.pdf.

[2] Balaras, C. A. et. al. Solar air conditioning in Europe - an overview, Renewable and Sustainable Energy Reviews 11 (2007) 299-314.

[3] Best, R. and Ortega, N. Solar refrigeration and cooling. Renewable Energy Volume 16; Issues 1-4, January-April 1999. p 685-690.

[4] Kim, D.S., Infante Ferreira, C. A. Solar refrigeration options - a state ofthe artreview. International Journal ofRefrigeration 31 (2008); 3-15

[5] Mittal, V et. al. The study of solar absorption air-conditioning systems. Available at: http://www.erc.uct.ac.za/jesa/volumel6/16-4jesa-mittal.pdf Accessed: 20-05-2013.

[6] Serban, A., Chiriac, F. Instalatii frigorifice (Refrigeration Plants). Agir Publishing House, Bucuresti; 2010.

[7] Ferreira, C.A.I. Thermodynamic and physical property data equations for ammonia-lithium nitrate and ammonia-sodium thiocyanate solutions. Solar Energy. 1984; 32(2):231-236.

[8] Kim, J.S., Park, Y., Lee, H. Performance evaluation ofabsoiption chiller using LiBr + H2N (CH2)2OH + H20, LiBr + HO (CH2)3OH + H20, and LiBr + (HOCH2CH2)2NH + H20 as working fluids. Applied Thermal Engineering 1999; 19(2):217-225.

[9] Saravanan, R., Maiya, M.P. Thermodynamic comparison of water-based working fluid combinations for a vapour absorption refrigeration system, Thermal Engineering. 1998; 18(7):553-568.

[10] Yokozeki, A. Theoretical performances of various refrigerant-absorbent pairs in a vapor-absorption refrigeration cycle by the use of equations of state. Appl. Energy 2005; 80(4):383-399.

[11] Muhumuza, R. Modelling, Implementation and Simulation of a Single-Effect Absorption Chiller in MERIT. A thesis submitted in partial fulfilment for the requirement of degree in Master of Science in Renewable Energy Systems and the Environment, 2010, University of Strathclyde, United Kingdom.

[12] Chiriac, F., et. al. Heat exchanger with micro-channel for absorption chillers, with ammonia-water solution, for small cooling power. 4th IIR Conference on Thermophysical Properties and Transfer Processes ofRefrigerants. Proceeding papers. Delft, The Netherlands, 2013.

[13] Ibarra-Bahena, J., Romero, R. J. Performance of Different Experimental Absorber Designs in Absorption HeatPump Cycle Technologies. A Review, Energies 2014, 7, 751-766; doi: 10.3390/en7020751.

[14] Herald, K. E. et. al.. Absorption Chillers and Heat Pumps. CRC Press, Inc.; 1996.

[15] Thermo Physical Properties ofRefrigerants. ASHRAE Handbook. 2005.

[16] Chiriac, F. Instalatii frigorifice, (Refrigeration Plants). Editura didactica si pedagogica (Didactical and Pedagogical Publishing House ), Bucuresti; 1981

[17] Klein S.A.: EES-Engineering Equation Solver. Educational Version Distributed by McGraw Hill, F-Chart Software.