SOLAR VENTILATION Prof. Ing. Karel Adámek, CSc., Ing. Miloš Pavlů, CSc., Ing. Milan Bandouch Introduction Designed solar panel uses well known principle of so-called Trombe wall (patent Morse from 1881). It have a shape of classical window, so that it is possible to mount it in standard window frame instead window sash, or anywhere as independent panel, for instance on the façade. The cross section of the structure is simple. From the outer (front) side remains the transparent glass wall, where the solar radiation is entering. The inner glass is replaced by a thermal insulating wall. Between those two walls is situated so-called absorber made from black metal plate, which is warmed by incoming solar radiation. In the lower part there is situated a well sized suction of outer air, which is warmed by its flow along warmed black metal plate up. The air flow is exhausted by an axial fan, the source of electric energy there is the photocell, situated on the lower front side of the panel. Through the fan the warmed air is flowing in ventilated and tempered room. In such manner the problem of unventilated and or uninhabited rooms (increased humidity etc.) is removed and by tempering of inhabited room the heating season is shorter etc. The system needs not any invoiced energy source. The Fig. 1-1 presents such windows panel. 1. Numerical model of the solar panel Realized numerical simulations [1] helped by the assessment of various design modifications of absorbers built-in in solar panels, with the aim of simple production and of the highest thermal efficiency, too. The subject of this paper is not the detailed assessment of individual tested variants, but the illustration of solutions and received results, only. There are presented results of one typical design, finally chosen for the realization regarding the manufacturing costs, heat output, shape, appearance etc. The geometry of the model is given by structural layout, defined dimensions are the width of 600 mm, height of 1200 mm and the thickness of 40 mm with inserted absorber. To keep the flatness at significant thermal dilatations some simple cross stiffeners are added The aim of the simulation is to explore /qualify the image of the flow field, therefore the heat source is simply defined in the absorber body as constant (700 W.m-2), though the real value is changing after the year period, weather, orientation toward the sun, etc. In prismatic model are used hexahedral elements of mesh. Regarding the geometric symmetry the half-model is solved and displayed, only. It is possible that in the system arises some horizontal and unsteady velocity component, flowing across the central symmetry plane. But such symmetry plane prevents such movement, so that the solution could not be fully exact. But with many solved cases of half-models many time could be saved during many prospecting and testing numerical simulations and the final solution, only, will be made as full checking model. 1.1. Results The result of numerical simulation is presented on cross sections of the model. In general, the scales on individual figures are different to get the maximum full contrast of images. The Fig. 2-1 shows the temperature field in two reference planes of the front view. On the front side the light is traversing the glass and the thermal energy is absorbed by the absorber, the cold air is coming from the bottom and is heated along the front side of the absorber. The back side is thermally insulated, the air flows from up to down and is heated further along the back side of the absorber. So the air temperature is increasing continuously and the pressure is decreasing continuously from the atmospheric value to the underpressure at the suction inlet of the fan. The same situation is visualized once again in the detail of the cross section in some height of the panel. The warmest surface is that of the absorber, the coldest air there is at inner surface of the front (outer) glass, in the back (here in the right) channel the temperature is higher than in front channel. The Fig. 2-2 presents the pressure field in the same planes. At the inlet (front, down) there is the atmospheric pressure, at the outlet (back, down) there is the underpressure given by used suction fan. The pressure is decreasing continuously due to the flow resistance. 2. Facade panel (Solar Wall) Analogous design was used for large full-area facade panel. Due to larger dimensions this system is equipped by a mains-operated axial fan and light plastic glazing is used. Warmed air is blowing in the adjacent space (residential, manufactural, store,...) for its tempering. In 3+3 panels of 1 m width and 5 m height the warmed air is flowing up. In the common central channel all flows are collected and took away into the tempered space - see the Fig. 3-1. Such layout is given by a real situation of the realized function model. From design or visual reasons the thickness of the central outlet channel should remain the same as of inlet channels. Of course, in such a manner arises higher pressure resistance in the outlet area. The model design remains as above, without next description. 2.1. Results Several geometric configuration of the façade were solved and tested, for instance some variation of the system Tichelmann (1861-1926), used for floor heating etc., typically after the Fig. 3-2. For required compact view of the façade - common inlet/ outlet horizontal channels are of the same cross section as individual vertical links - such layout is not suitable for intended use, because the pressure / flow balancing is not good here. To get well-balanced flows in individual channels, the model after the Fig. 3-3 was solved. The general problem of such designed system are the mutual contacts of individual vertical flows in the common upper channel, whereby the inner flows represent substantial obstacles for outer flows. Therefore the flow ratios among individual flows are very high, typically max/min = 2,93/1. The typical image of streamlines shows the Fig. 3-3. Therefore the upper ends of channels were furnished by cross partitions after the Fig. 3-4. Such partitions create shading of individual flows in vertical channels and their turning in the horizontal direction, so that their mutual interference is smaller. Together with slight enlargement of the common upper horizontal area the flow inequality is decreasing essentially, to the better flow ratio max/min = 1,17/1. But in such layout arises the problem with local overheating in upper corners, because such corners are not well flowed. The Fig. 3-5 shows the global temperature field with overheated upper corners. The problem was removed by small additional gaps in corners, thereby they are well flowed. The above mentioned considerations about pressure (flow) balancing in “cold” model of solar facade were completed by heat exchange as in the reality. Results of simulations in “hot” model confirm our expectation: in those channels, where the flow is smaller due to the increased flow resistance, the resulting temperature is decreasing. So that the resulting heat flows are well-balanced, their ratio max/min = 1,09/1, only. So it is not necessary to tune individual flows absolutely, because the resulting effect – air warming – is automatically well balanced and more, the individual flows of various temperature are mixed in the common outlet. 3. Operating point of the system All simulations above were made for “hard” source of the pressure, i.e. the pressure value is independent on the flow value. Regarding the real soft characteristic of used fan it should be to determine real flow parameters in the model (both air flow and pressure or the flow resistance respectively). The Fig. 5-1 shows the real working characteristic of the used fan or its substitution by straight line. Next Fig. 5-3 shows several real operating points of the system fan + panel, for various panel designs. They are defined as points of intersection of fan characteristic (decreasing line “vent”, similar to the Fig. 5-1) with individual panel characteristics, i.e. flow resistances of observed panels. They are received by repeated simulations /calculations of the volume flow for several values of the pressure gradient. Due to different internal arrangement of individual cases the characteristics are slightly different, too. The influence of the filter element permeability [3], enclosed in the air inlet, is not significant. 3.1. Internal air distribution The air warmed in above described system of solar panels can be distributed further in the adjacent inner space. They were used textile hoses with rows of small outlets along - they are light, simply transported and mounted, silent during the operation. Of course that such added flow resistance changes the operation point of the whole system panel + fan + hoses, that is why it is suitable to simulate it. Rows of small outlets along hose length are substituted by narrow gap of the equal cross area. 4. Experiments 4.1. Test chamber The first experiment was realized in thermally insulated testing chamber (the volume 6,5 m3 approx.). The used energy source is headlight 300 W in the distance of 1 m from the absorber surface of 0,7 m2). The Fig. 6-1 presents measured energy delivered into the chamber in the time. The temperature with panel signed 3K (own design) is increasing faster than with panel signed SF30 (another producer). In both cases there is evident an initial delay – warming of the chamber mass. By the estimation it could be said that heating outputs of installed panels are of about 50% or 100% respectively greater than used inner artificial heat source (i.e. bulb and fan, together 125 W approx.). Reaching of the steady state needs a long time, The steady state was predicted by using of suitable function with consecutive extrapolation. After removing of the initial convex part of the curve the suitable function (for blue curve) is y = a . (b – d.e -t/c), with parameters a = 9,06313, b = 3,17266, c = 1665,09, d = 3,16807. The limit value lim y for t→∞ is equal 33,4543 Wh, the value of 95% of limit (=31,7815 Wh) is reached after 5476 s, i.e. 1,5 hour approx., the value of 99% of the limit (= 32,7852 Wh) is reached in 7001 s, i.e. less than 2 hours. Remarks: 1) During the long-time experiment the parameters of the surrounding can be changed. 2) From result of numerical simulation on the Fig. 6-2 it is clear that the placement of the inlet of warmed air just at the bottom of the chamber is not right – the bottom wall is locally overheated. 4.2. Long-time measuring All numerical simulations above were made for constant heat source of 700 W.m-2 to ensure a good comparing of all solved cases. Really, the incoming energy is variable after the geographical latitude, spatial orientation of the surface, year and day period and weather. The Fig. 6-3 shows the graph of measured values during the period from 15.10.2013 to 31.10.2013, next Fig. 6-4 shows detail for one day. Conclusions Described system is a suitable example of application of renewable energy source, which is not only ecologic, but economic, too. Without supply of any invoicing source of energy it is possible to ensure continuously the aeration of unhabitated rooms and to remove problems of humidity, fug etc. In habituated rooms the system ensures the shortening of the heating season in the spring and autumn. The shape designed after real situation is suitable to balanced the flows / pressure resistances by small shape modifications. Relevant air warming is logically inverse proportional to relevant air flow. In the common outlet the individual air flows of different temperatures are mixed. To operate any system ecologic, only, but not economic, is nonsense and usually it masks any tendency of “privatization” of the national budget. Really it means on the basis of attractive formulations to get any dotation on timely unstable energy source and the neglect the care for necessary problem of investing for and operation of necessary reserve source. The interesting presented system is result of cooperation among the theory (author*), design (author***) and realization (author**). System is reliable, independent on invoiced energy sources and improves the living environment.
Posted on: Sun, 31 Aug 2014 17:21:13 +0000
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