Scholarly article on topic 'Solar Power Generation System with Low Temperature Heat Storage'

Solar Power Generation System with Low Temperature Heat Storage Academic research paper on "Agriculture, forestry, and fisheries"

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Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — Daniel Dragomir-Stanciu, Constantin Luca

Abstract The paper analyze a small power generating system that convert solar energy into electricity using an organic Rankine cycle. Solar thermal energy is stored at low temperature in a phase change material. The phase change material used is paraffin wax and the organic fluid is R134a. Was calculate the thermodynamic cycle and were established relationships to calculate the working agent mass flow and the necessary mass of phase change material, depending on the electric power produced.

Academic research paper on topic "Solar Power Generation System with Low Temperature Heat Storage"

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ScienceDirect

Procedía Technology 22 (2016) 848 - 853

Procedía

Technology

9th International Conference Interdisciplinarity in Engineering, INTER-ENG 2015, 8-9 October

2015, Tirgu-Mures, Romania

Solar Power Generation System with Low Temperature Heat

Storage

Daniel Dragomir-Stanciua*, Constantin Lucab

a"Petru Maior" University of Tirgu Mure§, Str.Nicolae Iorga, nr.l, 540088, Tirrgu Mure§, Romania bTechnical University "Gheorghe Asachi"of Ia$i, Bld D. Mangeron nr. 61, 700050, Iayi, Romania

Abstract

The paper analyze a small power generating system that convert solar energy into electricity using an organic Rankine cycle. Solar thermal energy is stored at low temperature in a phase change material. The phase change material used is paraffin wax and the organic fluid is R134a. Was calculate the thermodynamic cycle and were established relationships to calculate the working agent mass flow and the necessary mass of phase change material, depending on the electric power produced. © 2016 The Authors.PublishedbyElsevierLtd. 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 "Petru Maior" University of Tirgu Mures, Faculty of Engineering Keywords: low temperature solar heat storage. power generation; organic Rankine cycle.

1. Introduction

Converting solar energy into electricity can be achieved in solar power plants whose operation is based on the classic water-steam Rankine cycle or organic Rankine cycle. Because solar radiation intensity is variable, depending on annual cycles, daytime and weather, solar power plants must include thermal energy storage systems.

The first scientist who highlighted the need for solar energy storage, due to the solar radiation discontinuity, was Henry E. Willsie. Willsie build in 1904 the first solar power plants, equipped with flat solar collectors and thermal energy storage system [1].

* Corresponding author. Tel.: +40749768485 E-mail address: daniel.dragomir@ing.upm.ro

2212-0173 © 2016 The Authors. 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 "Petru Maior" University of Tirgu Mures, Faculty of Engineering doi:10.1016/j.protcy.2016.01.059

The advantages of thermal energy storage systems compared with other energy storage systems, mechanical or chemical, consisting of low environmental impact, ease of use and low cost system because of its simplicity constructive [2]. There are three main ways to store solar thermal energy: sensible heat storage, latent heat storage and thermochemical heat storage [3], [4].

For solar power plants that operate based on organic Rankine cycle, heat storage is done at low temperatures, often using latent heat storage. In latent heat storage systems, energy is stored during melting and restores during solidification of a phase change of a material [5]. Heat storage is based on the latent heat of the phase change material (e.g. paraffin wax, fatty acids, salt hydrate).

The paper studies a micro power plant using solar heat storage at low temperature (55-60°) in paraffin wax. Stored heat is converted into electrical energy in an organic Rankine cycle whose working agent is R134a. In the literature there are few studies on the production of electricity in an organic Rankine cycle based on solar energy stored at very low temperature. Most applications propose solar heat storage at temperatures over 100°C [6].

Starting from the parameters of thermodynamic Rankine cycle, were established relationships to calculate the working agent mass flow and the necessary mass of paraffin wax, depending on the electric power.

2. Solar power plant diagram and functional parameters

Schematic diagram of the solar power plant is represented in Fig.1. Solar energy from solar collectors is stored in paraffin wax storage tank. The thermal energy stored in paraffin wax is transferred to the organic fluid through a heat exchanger. Inside the heat exchanger the organic fluid vaporizes. The vapors are expanded in the turbine, which drives the electric generator. The exit vapor is condensed in the condenser and the pump increases the pressure of the liquid fluid before vaporizer.

The maximum temperature of the R134a vapors was adopted 50°C, depending on the paraffin wax melting range of 55-60°C . The minimum temperature is 22°C, in order to provide cooling of the condenser. The pressure at the turbine inlet is 13,18 bar and the minimum pressure at condenser is 6,08 bar. Thermodynamic properties of R-134a were determined according to [7].

Specific heat received by organic fluid from paraffin wax to vaporize is qj= 192,42 kJ/kg. Specific heat discharged to the condenser is q2 = 180,51 kJ/kg.

Specific energy produced by the turbine for 1 kg of working agent is 12,64 kJ/kg. The thermal efficiency of the cycle 6,19%. The values obtained for thermal efficiency are similar to those presented in the literature for organic Rankine cycle operating at low temperatures [8], [9].

Because this power system is indented for households and small consumers, electrical power was considered in the range of 0.2 - 4.0 kW.

3. Organic fluid massflow and phase change material mass calculus

To determine the overall dimensions of the power plant should be determined organic fluid mass flow and necessary mass of the phase change material.

The power produced by the turbine is calculated with equation:

PT = m -eT [kW] (1)

Where : m is the organic fluid mass flow [kg/s] ;

eT is specific energy produced by the turbine for 1 kg of working agent [kJ/kg] . Electrical power produced by generator is:

Pel = Pt ■ Лес [kW] (2)

In Eq.(2) Лес represents the efficiency of the electric generator; was adopted value by 0,94. From the previous equations can determine the relationship of organic fluid mass flow calculation according to the electrical power:

™ = ТГТ [kg/s] (3)

An important aspect of the construction of this solar energy group is phase change material mass, which will influence the size dimensions of the storage tank. It is considered that solar heat is delivered to phase change material wax until it melts completely and the heat transferred to organic fluid is only the latent heat of solidification.

Heat flow received by organic fluid during the time т is given by equation:

Qof = T-q [kJ] (4)

Heat flow delivered by phase change material in the same time period is:

Qpcm = qof/vsc [kJ] (5)

ц5С is heat exchange efficiency between phase change material and organic fluid, considered 0,8. On the other hand the heat from phase change material can be calculated using the equation:

Qpcm = MPCM ' r [kJ] (6)

Where: MPCM is mass of phase change material [kg];

r represents the latent heat of phase change material; for paraffin wax was considered 200 kJ/kg [10]. From Eq (4), (5), (6) can calculate the mass of phase change material in dependence on organic fluid mass flow:

MPCM = [kg] (7)

Using Eq.(3) and Eq.(7) can be obtained the relationship of the mass of phase change material depending on electric power

M _ 'Pel rt ,

MPCM = p ..„ [kg]

erVEG Vscr

From a practical standpoint, tank storage size and mass of phase change material must be adopted according to the average energy consumption of the user. Thus, we consider it is necessary to established a relationship allowing phase change material mass in dependence on a given amount of electricity.

The relationship of phase change material necessary mass depending on electricity produced by power plant, Wel,

Mpcm =

3600-q^-Wei erVEG"Vsc-r

4. Results

Based on Eq (3) was calculated the required massflow of R134a in dependence on the electrical power produced by the generator. The results are shown as a graph in Fig 2.

H 0.25

0.2 0.6 1 1.4 1.8 2.2 2.6

electric power [kw]

3.4 3.8

Fig. 2. R134a necessary mass flow

R134a mass flow range is from 0.017 kg/s for a power of 0.2 kW and 0.340[kg/s, for a power of 4.0 kW.

For paraffin wax mass calculus were considered 3 values of operating time of the micro power plant: t = 3600 s (1 h) t = 7200 s (2 h) t = 10800 s (3 h)

Results are presented in Fig.3.

Choosing electricity produced in range 0.5 to 12 kWh, was calculated with Eq.(9) paraffin wax necessary mass. The results are shown in the graph of Fig.4. From the graph represented in Fig. 4 can determine the mass of paraffin wax according to the average energy consumption. For example, a one family house has an average electricity consumption of 2 kWh/day. The mass of the paraffin wax should be 736 kg.

5. Conclusions

The obtained results demonstrate that it is possible to produce electricity in power plants with solar heat storage at low temperature.Eq. (8) and Eq. (9), allowing the calculation of the mass of paraffin wax on the electric power or electricity needed, it can be generalized to any type of phase change material used in latent heat storage and every type of working fluid used in organic Rankine cycle.

From a practical standpoint phase change material mass be as small as possible to minimize the investment and the overall dimensions of the power plant.

The ratio of the amount of heat introduced into the cycle and energy produced by the turbine per kg of working agent should be as small.

• 1 h 2 h •3 h

QV O- O- O- O- O- O- O-

Fig. 3. Paraffin wax mass in dependence on R134a mass flow and time R134a mass flow[kg/s]

4500 4000 3500

* 3000

I 2500

S 2000 £

E 1500

1000 500 0

0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 Energy produced [kWh]

Fig. 4. Paraffin wax mass in dependence on energy

References

[1] Canivan J. Solar Thermal Energy. Sunny Future Press 2005; p.26.

[2] Ibrahim H, Ilinca A, Perron J. Energy storage systems—Characteristics and comparisons. Renewable and Sustainable Energy Reviews 2008, vol.12 (5), , p.1221-1250.

[3] Pinel P, Cruickshank C.A, Morrison I.B, Wills A. A review of available methods for seasonal storage of solar thermal energy in residential applications. Renewable and Sustainable Energy Reviews, 15, 2011, p.3341- 3359.

[4] Dincer I, Rosen M, Thermal Energy Storage: Systems and applications, Ed. Wiley, London, 2002.

[5] Om Nayak, A, Gowtham, M, Vinod, R, Ramkumar, G. Analysis of PCM Material in Thermal Energy Storage, International Journal of Environmental Science and Development, Vol. 2, No. 6, 2011, p. 437-441.

[6] Tchanche BF, Lambrinos G, Frangoudakis A, Papadaki G. Low-grade heat conversion into power using organic Rankine cycles-a review of various applications. Renewable and Sustainable Energy Reviews, 15, 2011, p.3963-3979.

[7] ASHRAE Handbook Fundamentals , American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 2009, ISBN 978-1933742-54-0.

[8] Khennich, M, Nicolas G. Optimal Design of ORC Systems with a Low-Temperature Heat Source, Entropy, 14, 2011, p.370-389.

[9] Velez F, Segovia JJ, Carmen Martin M, Antolin G, Chejne F, Quijano A.Comparative study of working fluids for a Rankine cycle operating at low temperature, Fuel Processing Technology, Vol. 2, No. 6, 2011, p. 437-441.

[10] Ukrainczyk N, Kurajica S, Sipusic J. Thermophysical Comparison of Five Commercial Paraffin as Latent Heat Storage Materials, Chem. Biochem. Eng. Q. 24 (2), 2010, p.129-137.