saturs - Latvijas Lauksaimniecības universitāte

U. Iljins, I. Ziemelis The Optimization of Some Parameters of a Flat Plate Solar CollectoryQT 0α glα absλ aλq iT IIIT IIδ 3δ 2x 0iλ iT Iδ 1xα rLT 0Fig. 1. The scheme of calculation:T I – temperature in the heat insulation layer, o C; T II – temperature in the absorber plate, o C;T III – temperature in the space between the glass cover and absorber plate, o C;T o – the ambient air temperature o C; δ 1 – the thickness of the heat insulation layer, m;δ 2 – the thickness of the absorber plate, m; δ 3 – the distance between the glass cover and absorber, m;λ i , λ – the heat transfer coefficients of the heat insulation material and absorber plate material,W⋅(m⋅K) -1 ; λ a – the equivalent heat transfer coefficient of the air layer between the glass cover and theabsorber plate, W⋅(m 2 ⋅K) -1 ; α gl – the contact heat transfer coefficient from the glass to air, W⋅(m 2 ⋅K) -1 ;α abs – the contact heat transfer coefficient from the absorber to air, W⋅(m 2 ⋅K) -1 ;α r – the contact heat transfer coefficient from the rear surface of the collector to the ambient air,W⋅(m 2 ⋅K) -1 ; Q – the specific power of the absorbed solar energy, W⋅m -2 ;x oi – the co-ordinate of the heat transfer medium tube, m.In compliance with the task of the investigation, the temperature distribution in the following three layers hasto be computed:T 1– air temperature in the space between the glass cover and absorber, o C;T 2– the absorber plate temperature, o C;T 3– the temperature in the heat insulation layer, o C.On the rear side of the collector where y=0, the heat convection takes place, therefore the heat flow can beexpressed by the equationλ ∂TIi= αr( TI− Ty=0 0)∂y, (3)y=0whereλ i– the heat transfer coefficient of heat insulation material, W ⋅ (m ⋅ K) -1 ;α r– the contact heat transfer coefficient from the back side surface of the collector to ambient air, W⋅ (m 2 ⋅K) -1 ;T 0– the ambient air temperature, °C.The heat convection takes place also from the front surface of the collector, where y=δ 1+δ 2+δ 3. The correspondingboundary condition for this is analogous to formula (3):− λa∂T∂yIIIy=δ1+δ2+δ3= α− T0) , (4)68 LLU Raksti 12 (308), 2004; 67-75 1-18f(TIII y=δ1+δ2+δ3whereλ a– the equivalent heat transfer coefficient of the air layer, W ⋅ (m ⋅ K) -1 ;α f– the contact heat transfer coefficient from the front side surface of the collector to ambient air, W ⋅ (m 2 ⋅ K) -1 .On the border of the heat insulation layer and the metal sheet, the temperature and heat flows are equal,which can be written as following: