Simulation and Comparative Study of a Hybrid ...

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ScienceDirect Energy Procedia 57 (2014) 2646 – 2655

2013 ISES Solar World Congress

Simulation and Comparative Study of a Hybrid Cooling Solar – Gas with Heat Storage Pando Martínez GEa,b*,Sauceda Carvajal Dc,Velázquez Limón Na,Luna León Ad, Moreno Hernandez Cb a

Centro de Estudios de las Energías Renovables, Instituto de Ingeniería, Universidad Autonóma de Baja California, Calle de la normal s/n col. Insurgentes, Mexicali, B.C. 21280, México. b Instituto Tecnológico de Hermosillo,Av. Tecnológico s/n col. El Sahuaro, Hermosillo, Son., 83170, México . c Facultad de Ingeniería, Universidad Autonóma de Baja California, Calle de la normal s/n col. Insurgentes, Mexicali, B.C. 21280, México. d Facultad de Arquitectura, Universidad Autonóma de Baja California, Calle de la normal s/n col. Insurgentes, Mexicali, B.C. 21280, México.

Abstract In this paper, we show a comparison between an absorption chiller with direct fire activated and a hybrid

solar-gas absorption chiller, with a cooling capacity of 17.6 kW each (5 TR) installed in a residence located in Mexicali, BC Mexico, to do simulations were performed during the summer of the town using dynamic simulation through the environment TRNSYS 16 mainly obtained LP gas consumption and the reduction in the consumption of it to use solar energy as a heat source in the generator. To produce the required heat used eight parabolic trough collectors configuring them as follows: 4 collectors in parallel and in series 2 collectors results show a reduction in LP gas consumption of approximately 24% during the period evaluated. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

© 2013 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES Selection and/or peer-review under responsibility of ISES.

Keywords: Hybrid solar-gas, TRNSYS, LP gas consumption, Absorption chiller, Parabolic trough collector

* Corresponding author. Tel.: +52-686-566-41-50; fax: +52-686-566-41-50. E-mail address: [email protected]

1876-6102 © 2014 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/3.0/). Selection and/or peer-review under responsibility of ISES. doi:10.1016/j.egypro.2014.10.276

Pando Martínez Ge et al. / Energy Procedia 57 (2014) 2646 – 2655

1. Introduction The human beings throughout history have sought to live comfortable conditions for that, mainly in arid areas, semiarid and desert air conditioning has been used for such conditions. In most cases, this need has been met based on the use of systems based on the technology of mechanical vapor compression, which use a large amount of electric energy as driving force, which in Mexico is causing serious problems in the electricity sector, the environment and society. In these areas of the country electricity demand of the season from winter to summer is as much as three times higher primarily due to air conditioning [1]. Bringing an impact on the household economy in the summer months. Solar Heating and Cooling systems are well-known renewable energy plants which provide heating and cooling energy, by converting the solar irradiation incident on a solar collector field. SHC systems have been investigated since 1970, when a number of demonstration projects were conducted in several countries. However, such demonstration prototypes did not reach the commercial stage of their development, mainly due to their high initial costs [2]. Thus, SHC technology was abandoned for some decades. Then, in the last few years, a new impulse on this topic has been given, due to the increasing costs of fossil sources and to the enhanced interest in environmental issues. In fact, several institutions are presently involved in R&D activities in this field and a lot of innovative demonstration projects have been developed [3].The market potential of SHC systems is very promising, also as a consequence of the dramatic growth in the energy demand for cooling, especially in residential, commercial and office buildings [4]. The energy demand associated with air-conditioning most industrialized countries has been increasing noticeably in recent years, causing peaks in electricity consumption during warm weather periods. This situation is provoked by improved living standards and building occupant comfort demands and architectural characteristics and trends such as an increasing ratio of transparent to opaque areas in the building envelope [5]. The above, along with refrigeration requirements in the food processing field and the conservation of pharmaceutical products in developing countries are leading the interest in airconditioning and refrigeration systems powered by renewable energies, especially solar thermal, which work efficiently and in certain cases, approach competitiveness with conventional cooling systems. Solar thermal systems in addition to the well-known advantages of renewable resources (environmentallyfriendly, naturally replenished, distributed…), are very suitable for air-conditioning and refrigeration demands, because solar radiation availability and cooling requirements usually coincide seasonally and geographically [6, 7]. Solar air-conditioning and refrigeration facilities can also be easily combined with space heating and hot-water applications, increasing they early solar fraction of buildings [8]. Mathematical models and simulations of different complexity have been developed to simulate and assess the performance and characteristics of the combined solar collector and single-effect absorption cooling machines. New design options of the solar collector module have been developed. Some software packages like TRNSYS and EES were found to be useful for authors to model and optimize their systems in concern [9]. Florides et al. [10] modeled and simulated a LiBr–water 11 kW cooling capacity solar driven absorption cooling machine which could cover the cooling load of a typical model house in Cyprus. The system is modeled with the TRNSYS simulation program to get the optimum system parameters, and the total life cycle cost of a complete system including the collector and the absorption unit. An integrated transient simulation program is developed by Joudiand Abdul- Ghafour [11] for simulating the Iraqi solar house lithium bromide–water absorption cooling system using TRNSYS. The authors concluded that the solar cooling system can be simulated successfully by using TRNSYS with comparable results with actual solar cooling systems. Moreover, using the cooling f-chart and the storage

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size design chart, or their related equations, simplifies the designer task for predicting the solar fraction for solar cooling systems for any type of building, collector, meteorological conditions and locations. Different mixtures as the working fluid in the use of absorption cooling technology, however, are basically two most used by the manufacturers of air conditioning by absorption. One is a solution of ammonia and water being ammonia refrigerant and water the absorbent. Some of the most important manufacturers in this group are the equipment signature Cooling Technologies and Robur. The other solution corresponds to lithium bromide and water where it acts as a refrigerant being lithium bromide absorber major brands that use this type of solution are Carrier, Thermax, Trane, York and Yazaki. The equipment that use an ammonia-water solution with a capacity of 17.6 kW have a COP of between 0.68 to cooltec5 brand of Cooling Technologies for Robur and 0.7 according to the technical specifications provided by the manufacturer, however the cooling capacity is determined with an outside temperature of 35 ° C. A study by Boian sample is reduced as the cooling capacity for different temperatures of the chiller output relative to external temperatures [12]. Considering that Mexicali, B.C. Mexico has temperatures above 45 º C in the summer season; we conducted a study of the operational behavior of a unit of its kind installed in this city. We used the simulator TRNSYS 16 obtaining the temperature plays an important role in reducing the cooling capacity and COP in their respective absorption chiller direct fire tested, however, and despite some days the unit does not provide nominal cooling capacity, can meet and satisfy the needs of air conditioning during the summer season and that only 3.9% of the time the housing temperature was one degree Celsius above the temperature set on the "set point "[13].

Nomenclature

PTC

Parabolic trough collector

SHC

Solar heating and cooling

TRNSYS

Transient systems simulation

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(a)

(b)

Fig. 1. (a) Absorption Refrigerant Cycle; (b) Schematic of a parabolic trough collector 2. System description. The thermodynamic cycle absorption cooling, as the compression is based on the need of the fluid used as a refrigerant to obtain a cooled liquid heat to pass from the liquid to vapor. From this perspective solar powered cooling systems as a green cold production technology are the best alternative. Absorption refrigeration is a mature technology that has proved its applicability with the possibility to be driven by low grade solar and waste heat [9]. 2.1 Absorption chiller The operation of an absorption cooling machine fig. 1 (a) can be summarized as follows: the waterammonia solution is heated by a burner in the generator (in this case LP gas) where the ammonia evaporates, separating the water. Then it is driven through a battery of tubes (condenser) where it is cooled thanks to the passage of an air flow at room temperature produced by an external fan and passing again condenses to liquid. The resulting liquid ammonia is fed to a heat exchanger (evaporator) where the evaporation pressure difference and the absorption of heat from the external circuit (water coming from the cooling coil) as it cools. The ammonia is led to the absorber, where it meets the remaining water after separation in the generator. At this time, absorption occurs (ammonia vapor is absorbed by the water in the liquid solution from the generator), a process that gives its name to the duty cycle of the application, and later restart the cycle generator. 2.2 Solar collectors A solar collector, the special energy exchanger, converts solar irradiation energy either to the thermal energy of the working fluid in solar thermal applications, or to the electric energy directly in PV (Photovoltaic) applications. For solar thermal applications, solar irradiation is absorbed by a solar collector as heat which is then transferred to its working fluid (air, water or oil). The heat carried by the working fluid can be used to either provide domestic hot water/heating, or to charge a thermal energy storage tank from which the heat can be drawn for use later (at night or cloudy days) [14].

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2.3 Parabolic trough collectors Parabolic trough collectors can concentrate sunlight with a concentration rate of around 40, depending on the trough size. The focal line temperature can be as high as 350 ºC to 400 ºC. The key component of such collectors is a set of parabolic mirrors, each of which has the capability to reflect the sunlight that is parallel to its symmetrical axis to its common focal line. At the focal line, a black metal receiver (covered by a glass tube to reduce heat loss) is placed to absorb collected heat fig. 1 (b). Parabolic trough collectors can be orientated either in an east–west direction, tracking the sun from north to south, or a north–south direction, tracking the sun from east to west [14]. 2.4 Solar thermal energy storage After the thermal energy is collected by solar collectors, it needs to be efficiently stored when later needed for a release. Thus, it becomes of great importance to design an efficient energy storage system. There are three main aspects that need to be considered in the design of a solar thermal energy storage system: technical properties, cost effectiveness and environmental impact [14]. 2.5 Description of system operation The solar powered absorption cycle is divided into two subsystems: the solar sub-system and the cooling sub-system. The former includes the solar collector array, a storage tank and two pumps. The cooling sub-system consists mainly of the absorption cycle, one pump, a water-to-air heat exchanger and the conditioned space. The solar sub-system is composed of eight PTC’s which supply energy to an organic synthetic heat transfer fluid (Dowtherm A), which is stored in a tank of 0.5 m3 which takes the energy required by the absorption chiller. The collectors are arranged in an array of four collectors in parallel and in series 2 collectors oriented east-west to north-south track. The cooling sub-system is composed of the absorption unit which can operate with thermal oil from the storage tank or LP gas. The way the absorption chiller operates as follows: The thermal oil supplies the quantity and quality of energy required for the absorption chiller which is 23.74 kW otherwise the absorption chiller operates with LP gas. The cold water produced in the absorption chiller is fed through a variable flow pump to the cooling coil which removes heat from the conditioned space. The on and off the absorption chiller is based on the temperature inside the conditioned space.

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PTC Type 536

Weather Data Type 109

Psychrometrics Type 33

Calculate cloudiness factor Type 69

Storage Tank Type 4a

Air Cooled Chiller Type 655

Cooling Coil Type 508

Pump Type 114

Pump Type 114

Pump Type 110

Controller Type 2

Controller Type 2

Controller Type 2

Conditioned Space Type 56

Controller Type 22

Information Control signal Data

Fig. 2. System information flow diagram 3. System simulation. The latitude and longitude of Mexicali are 32.63ºN and 115.45ºW, respectively. Account with temperatures above 45 º C in the summer season, so you have a reduction in cooling capacity and its respective COP absorption chiller, however, and despite some days the chiller is not provide capacity nominal cooling, can meet and satisfy the needs of air conditioning during the summer season and that only 3.9% of the time the housing temperature was one degree Celsius above the temperature set on the "set point" which is 25 ° C [13].

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The complete solar absorption system is simulated using TRNSYS program. The system model consists of many subroutines, called Types. Each Type would represent a component in the system. Once all the Types have been selected, they are connected to each other to form the complete system. In order to track the flow of information between the various components, an information flow diagram for the whole system is created as shown in Fig. 2. The main objective of this work is to determine the amount to be used lp gas in the hybrid solar-gas absorption chiller when using a PTC arrangement whose parameters are found in Table 1, so you should determine the amount of energy provided by the storage tank, for it basically determined the conditions to be met in order to make use of the energy stored in the tank, which are mentioned below: ‚ ‚ ‚

The absorption chiller should be on. The storage tank temperature must be greater than 190 ° C. The heat supplied by the synthetic organic heat transfer fluid whose flow is 1094 kg / h should be greater than 23.74 kW.

If these conditions are met then the absorption chiller using solar energy works, otherwise require the absorption chiller LP gas supply for operation.

Table 1. Parameters for solar collectors Parameter Number of collectors in series Number of collectors in parallel Aperture area Concentration ratio Intercept efficiency (FrTan) Efficiency slope (FrUl) Fluid specific heat

Value 2 4 2.84 34 0.8 9.98 1.954

Unit m2 kJ/hr m2 K kJ/kg K

4. Results and discussion In this study the absorption chiller during operation can use solar or fossil fuel energy in the case of using the solar energy that is captured through eight PTC’s which heat thermal oil is stored in a tank and then sent to the generator to evaporate the ammonia water solution, where the energy stored in the tank does not meet the requirements in terms of quality and quantity refers the equipment operates through a direct fired generating the which uses LP gas as an energy source. Table 2 and Figure 3 shows the energy requirements of the specific absorption chiller in the simulation for the months of May to September in case you only use direct heat generator in case of hybrid solar-gas. While in Table 3 and Figure 4 can be seen LP gas consumption for both cases and percentage savings in monthly and annual average for the absorption chiller operate in a hybrid solar-gas. The calorific value used is 48 MJ / kg [15].


Table 2. Chiller energy requirements Month May Jun July August September Annual

Fire Direct (kWh) 9576 12249 16974 16926 13128 68844

Hybrid solar-gas (kWh) 6623 9733 12867 12534 10279 52036

Table 3. Chiller gas consumption Month May Jun July August September Annual

Fire Direct (kg) 717.53 918.68 1273.05 1269.45 984.60 5163.30

Hybrid solar-gas (kg) 496.73 729.98 965.03 940.05 770.93 3902.70

Fig. 3. Energy consumption

Saving (%) 30.77 20.54 24.20 25.95 21.70 24.41

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Fig. 4. Gas consumption The savings obtained in May is higher due to the useful energy gain of the collectors, which means you can use it for longer the stored energy in addition to that due to ambient temperatures that occur in that month the equipment does not demand the same amount of energy that the subsequent months, the useful energy gain of the collectors in kWh can be seen in the table 4. Table 4. Useful energy gain of the collectors Month May Jun July August September

Useful energy gain (kWh) 16008 14974 17980 17520 13907

5. Conclusions The absorption chiller can be operated in a hybrid solar-gas bearing savings of approximately 24% in LP gas consumption in the period under review to use eight PTC in an arrangement of 2 collectors in series and 4 in parallel. Can be simulated behavior of the two proposed subsystems together by TRNSYS.

Acknowledgements The autor wishes to aknowledge to Instituto Tecnologico de Hermosillo and the CONACYT for their support troughout the project CB-2011-01, 167794 and the scholarship for doctoral studies. .


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