Variables Optimization of Building Air Conditioning System

Transcription

1 Variables Optimization of Building Air Conditioning System MARIUS-CONSTANTIN POPESCU CORNELIA AIDA BULUCEA GHEORGHE MANOLEA CRISTIAN VLADU Faculty of Engineering in Electromechanical, Environment and Industrial Informatics University of Craiova, ROMANIA Abstract. A heating and climatizer system based on selective absorption of solar energy by a selective collector. The experimental study shows that the performance of this system depends on several variables: the nature of the colporteur fluid, the flow of the fluid circulation, the collector's surface area and the circulation mode (opened or closed circulation). The system was tested inside a building which contained a strawberry crop. A clear improvement of the performance has been achieved for an open circulation with appropriate working variables. The experimental results showed precocity of 14 days of the strawberry crop in comparison to a standard building. The variables of the experiment that can be controlled are: the flow of evacuated fluid, the entrance flow, the temperature of the fluid and the positioning of the transducer/ hole test. However, the measurement of these variables is affected by errors which cannot be completely eliminated. Key-Words: Modelation and optimization, Solar energy, Heating and climatisation. 1 Introduction In the European Community, agricultural buildings exceed 6000 ha and consume approximately 2% of Europe's total energy budget for heating and cooling [18]. Similarly, in Balkan European countries, buildings equipped with heating systems consume an average of 9.5 l of fuel per year per m 2 [3]. The buildings equipped with heating systems exceed 66 ha, approximately 10% of the total building surface and needs the equivalent of 300 Euro per ha per day during the winter [5]. This, however, represents 35% of the overall operational cost of the building. Consequently, this cost has been proved to be prohibitive and does not permit the use of heating systems in many applications. To overcome these problems, alternative heating technologies, with low cost, are required. This paper presents the thermic and agricultural results of a building heating and climatisation system based on selective absorption of the solar radiation by a semi-transparent collector. It presents a new concept of a heating system based on a previous work [6-8], [16]. 2 Material and Method 2.1 System Principle The principle of the system is based on selecting only the part of the solar radiation spectrum which is useful to photosynthesis (Fig.1). The remainder of the total spectrum energy is absorbed by the caloporter fluid moving around inside the collector and carried up to the storage tank. Fig.1: Spectral solar radiation (1) and spectral absorption of the chlorophyl (2). 2.2 System Operation The system has been set up to function according to temperatures allowing optimal plant growth. Time intervals during which the greenhouse environment was to be controlled have been determined. These were a storage phase from 10 h to about 18 h, a static ISSN: ISBN:

2 phase from 18 h to 24 h and a restitution phase from upon 24 h and above. Moreover two modes of circulation have been tested: - closed circuit: the pump moves the caloporter fluid from the storage tank through the collector and brings it back to the same tank; - open circuit: in this case the pump moves the fluid, during the day, from the cold tank to the storage tank through the collector. 2.3 Experimental Device The heating system is made of a collector socketed plan made of transparent polyethylene [2]. Communications between the socket have been arranged in such a way that a fluid moves in a worm in the whole collector. Three collectors (size 2.5 m 2 for each) were located in the experimental building at one metre from the ground and were connected to a storage tank by means of driving back pump (Fig. 2). a submersible electrical pump P3, which has on the CR3 upsetting pipe a tap R3. A constant level parallelepiped tank has at the entrance a conducting device that helps direct our work agent, which can be replaced either by pure water (tap R3) or by salt water (tap R4). An evacuation circuit, which has an electrical pump P1 identical to the one in the pure water circuit, has on the upsetting pipe CR1 a tap R1. The model corresponds to the simplified configuration of an air conditioned room. The measurements are done in a median plan. The perturbation generated by the hole introduced in the electrolyte is neglected and the signal from the conduction meter is accounted by a numerical voltmeter. For a scale factor coequal to 20 and respecting the Reynolds similitude, we obtain an input liquid flow of about 7,5 l.min -1. Fig.2: Experiment greenhouse with selective collector. The experimental connection without a collector (Fig. 3a) is sketched in Fig. 3b. A pure water circuit, which has a tank- RE1, an electrical pump- P2, has on the upsetting pipe- CR2 a tap- R2 and a water measuring device- AP, which measures the flow of water. A salt water circuit which contains a tank RE4, equipped with an all or nothing tap- R4 (the moment when the tap is opened corresponds to the moment when the pollutant is introduced into the model, starting the signal acquisition). The liquid travels from the tank to the model by falling; the flow obtained by fall is the same as the flow which a submersible electrical pump P3 pumps when the tap R3 is open. A second circuit of pure water allows, at any given moment, the replacement of the salt water flow with a pure water one. The circuit is made out of a) b) Fig.3: Experiment greenhouse without collector: a) overview; b) sketch. ISSN: ISBN:

3 The complete determination of the error was not possible. Thus, some hypothesis have arose: the error due to the positioning of the hole is negligible; the hole does not perturb the flow of liquid through the model. The inconveniences of the installation arise from the fact that we work in an open circuit. On the other hand, during work we can observe water bubbles that travel to the superior part of the model (in order to resume the measurements for a stationery system, we must expect until the bubbles disappear). The identification operation has unfolded in the time domain. So that the model might have a conduct similar to that of the process (the errors that occurs between the model and the process must be very small), we have used the medium square error. The obtained parameters are optimal because they have been obtained nu minimizing error criterion [9-10]. The influence of the sampling period has been studied; the noises from the transitory concentration curves have been eliminated (through a mathematical procedure) and the quality of the results obtained for both the transitory and stationary system have been verified. The conclusions were: a strong uncertainty in the inferior region of the model (given by the measurement of the flows) and a relative uncertainty of 5% for the rest of the model (given by locating the measurement point); in the attempts to reproduce the measurements in the transitory system, the uncertainty given by the measurement of the temperature and concentration in the inferior region of the model have been ignored, which have given birth to a systematic error; the big uncertainty in the evolution of the temperature and concentration arises, because the calculus is done with a limited number of points and because there was no filter to eliminate the noise. Fig. 4: Spectral absorption of methylene solution (0.03 g/l concentration) (1) and of chlorophyl (2). 3.2 Effect of Circulation Mode on Energy Captivated The closed circuit yielded a progressive rise of the fluid entrance temperature and then a fall of the power which is vehicled by the collector for any circulating flow. We observed that the vehicled power decreases when the fluid entrance temperature Te keeps increasing, in spite of the fact that the radiation received by the collector plan is growing (Fig.5). 3 Results and Interpretations 3.1 Caloporter Fluid The fluid constitutes the dynamic element of this system. In fact, during its circulation it exchanges heat with the greenhouse surroundings and, because of its colour, acts as a thermic screen for the plant. Particular attention is paid to the choice of fluid. In addition to selective absorption, the fluid must respond to other criteria like non-toxicity, chemical stability and low cost. One detailed analysis of several organic and inorganic solutions has been carried out and has led to the use of blue methylene at a concentration of 0.03 g/l (Fig.4). Fig.5: Power vehicled Pv, fluid entrance temperature Te and solar radiation on the collector G, during the storage duration (closed circuit). 3.3 Effect of Circulation Mode on the Storage Duration The storage duration corresponds to the period when the fluid output temperature Ts is greater than its input temperature in the collector Te. The analysis of Fig.6 which has been obtained with a closed circulation of the caloporter fluid, shows that the storage time has, relatively, a high value for small ISSN: ISBN:

4 flow of circulation ( /s). When the flow increases, the fluid input and output temperatures seem to quickly come close together [16]. Contrary to the closed circuit, the open circuit allows the system to use the complete day solar radiation. However, in the case of the open circuit, the storage time is independent of the fluid flow because the fluid input temperature Te, coming from the cold tank, remains lower than the output temperature Ts. Thus, storage time is maintained for the whole day's sunstroke period and the collector output keeps a value which is relatively constant [4], [17]. We should observe the flat nature of the curve around this value which offers an important interval for the regulation of the optimum flow. Fig.7: Collector efficiency for several flows. Fig.6: Fluid entrance temperature Te and fluid output temperature for the closed circulation. 3.4 Optimisation of Flow for Open Circulation In order to optimise the fluid flow in the selective collector with 2.5 m 2 surface area, we have evaluated its efficiency for several flow values; this efficiency is give by the expression: 3.5 Effect of Collector Surface on Efficiency Measurement of the temperature for each square metre of the collector and for several flows showed that the temperature elevation is perceptible for the first square metre (Fig.8). Beyond this, the elevation temperature is as slow as the flow is small. Figure 8 illustrates the energy spread by each square metre of the collector and its corresponding efficiency [1]. We note that the contribution of the first square metre is the most dominant. Indeed, the measured temperature difference for the first square metre is the most increased. This is because the entrance temperature of the caloporter fluid is small for the first square metre. On the other hand, the entrance temperatures for the second and for the third square metre increased progressively [11]. Ev µ s =, (1) G where G is the global solar radiation incident on the collector's plan set inside the greenhouse and Ev is the vehicled energy by square metre of the collector during the storage time; given by ( Ts Te) t+t dcp dt Ev = t, (2) S where d, is the circulation flow in the collector (1/s), S is the area of the collector (m 2 ), Te and Ts are, respectively, the input and output temperatures of the fluid ( 0 C) and dt the storage duration; from the curve of Fig.7, the flow which optimises the energy vehicled by the collector is /s. Fig.8: Fluid elevation temperature inside the collector for several flows. ISSN: ISBN:

5 3.6 System Optimum The system which operated with the optimum parameters was: caloporter fluid: methylene blue with 0.03 g/1 concentration; circulation mode: open circulation; circulation flow: l/s; collector surface: does not exceed 2 m 2 ; storage capacity: given by M = d. t, where d is the circulating flow and t is the storage duration which corresponds to the day duration for open circulation. During the winter, the average value for storage duration is: t - 9 hours, therefore M=355 1 for each collector. Thus, the total capacity for the three collectors is about 1 m 3 : collector efficiency: with this optimal variable µ c =49%; system efficiency: this represents the part of the incidental energy on the collector plan located inside the greenhouse which is stored in the tank. It is denned by the expression: M Cp T µ s = St, (3) G where M is the total storage capacity, T the storage tank's temperature rise during the storage period and St the total area for the three collectors. For many clear days, the average value obtained for us is 43%. Air temperatures are measured before and after the coil by retrieving the air from 20 points across the channel. Air flow is measured by means of calibrated nozzles. Water flow and temperatures are measured by a magnetic flow meter and calibrated thermocouples. The thermal power exchanged is therefore compared on both air and water sides: the difference is typically lower than 1 % (otherwise test is repeated). 4 Conclusions The trials on the models can offer information regarding the assays of calculus methods and solutions that cannot be offered by existing theory. We can easily say that the designing of models is a method of resolving problems that can help us reproduce, on a model, the phenomenon that we are studying. Simulating the phenomenon on a computer is another great method. Both these methods represent a powerful learning tool. For example, the climatic processes that take place in a closed area are complex and they cannot fully comprise the fluid movement phenomenon with heat and mass transfer [12-15]. The results obtained have shown that the system performance depends essentially on the caloporter fluid's circulating mode through the selective collector and on the circulating flow. The open circuit is the most suitable when used to exploit the maximum daily solar radiation. The optimal flow depends on the area of the collector surface. For a surface not exceeding 2 m 2, the optimum debit is about l/s. For a sunny day, it is recommended to use a high flow value during the day to vehicle the maximum surplus energy, this favours the greenhouse climatisation, and to use a low debit value during the night so that the stored energy will cover the whole critical period where heating is necessary. For a cloudy day, the flow must be low or even stopped (when the building temperature is below 10 C), to rise the plant temperature and ensure good growth. In that case a complementary heating system is to be considered. Elsewhere, in spite of the cold period when the plantation of the strawberry crop has been carried out, the building equipped with this solar heating system which operates with optimal parameters has shown precocity of 14 days of the strawberry crop in comparison to the control greenhouse without any heating system. References [1] Alsac, O., Vempati, N., Stott, B., "Generalized State Estimation," Proceedings of the PICA Conference, Columbus, OH, 1997, pp [2] Bulucea, C.A., Popescu, M.C., Manolea Gh., Patrascu, A., "Interest and Difficulty in Continuous Analysis of Water Quality", Proceedings of the 4 th, WSEAS International Conference on Energy & Environment, Feb.22-23, 2009, pp [3] Caouris, Y., Kittas C. and Santamouris M., "Solar & Wind", Techn. 6, 1989, pp.225. [4] Dahman, A. S. and Tadili, R., "Conception and study of a solar heating and climatisation system for a greenhouse", Renewable Energy Congress 5, 1994, pp [5] Lazrak, R., Mouklisse, P. and Tanouti, B., Applications du stockage de l'energie solaire au chauffage des serres agricoles, "Sechoirs solaires et systemes photovoltafques". Ed. Toubkal, Morocco [6] Fourcy, A. and Freychet, A., "Serre a climatisation solaire", Bull. Cofedes 6, Paris, pp [7] Fohr, J. and Thierny, R., "Evaluation des performances d'un double systeme de recuperation de ISSN: ISBN:

6 chaleur solaire d'une serre horticole experimentale", Rev. Ptasticulture 65, 1985, pp [8] Manolea, Gh., Popescu, M.C., Nedelcut, C., "Modelarea Pi simularea mediului din spaqiul de culturr a ciupercilor Pleurotus", Proiect Cercetare de excelenqr: Biotehnologii inovative pentru obqinerea de alimente funcqionale din ciupercile genului Pleurotus Pi pregrtirea exploatrrii industriale BIAF, [9] Popescu, M.C., "Modelarea Pi simularea proceselor", Editura Universitaria Craiova, 2008, pp [10] Popescu, M.C., "Estimarea Pi identificarea proceselor", Editura Sitech, 2006, pp [11] Popescu, M.C., "Approche systemique en genie automatique", Analele UniversitRQii din Craiova, HorticulturR, Biologie, Tehnologia prelucrrrii produselor agricole, Ingineria mediului, Vol. XII (XLVIII), Craiova, 2007, pp [12] Popescu, M.C., "The implementation of the predictive control of the thermic phenomenon inside rooms", International Carpathian Control Congress, Miskolc-Lillafured, Budapesta, 2005, pp [13] Popescu, M.C., PetriPor, A. "Models for optimal control of the air-conditioning variables from passive ventilated rooms", International Symposium on System Theory 12, Vol.1, Editura Universitaria, Craiova, 2005, pp [14] Popescu, M.C., PetriPor, A., "The experimental Investigation of heat transfer and friction losses interrupted and way fins for fin-and-tube heat exchangers", Tomul LII(LVI), Fasc.5C, ElectrotehnicR, EnergeticR, ElectronicR 4 th International Conferecnce on Electrical And Power Engineering, EPE 2006, 2006 pp [15] Popescu, M.C., PetriPor A. and Drighiciu, M.A., "Modelling and Simulation of a Variable Speed Air- Conditioning System", International Conference on Automation, Quality and Testing, Robotics, Proceedings, Cluj-Napoca, 2008, pp [16] Popescu, M.C., Balas, M.M., "Thermal Consumptions : Control and Monitoring". 3 rd International Workshop aon Soft Computing Applications, Szeged-Arad, 2009, pp [17] Tadili, R., Dahman, A.S., and El kiri, "Serres: Tabsorption selective de l'energie solaire", Rev. Horticole 347, 1994, pp [18] Von, Zabeltitz, C., "Energy conservation and renewable energies for greenhouse", FAO, ISSN: ISBN: