Description of calculation methodologies

In this work only thermal solar systems which are pre-heat or solar only systems for preparation of hot water are considered, since this is the case where the Portuguese regulations [1] imposes a specific calculation methodology [2].

1.1 European Standard EN 15316 (part 4-3) Methodology

The objective of this series of European Standards is the calculation of system energy requirements and system efficiencies. According to the standard, for thermal solar systems, the heat balance corresponds to the schematic representation of Fig.1 (for pre-heat/solar only systems). In this figured the terms relevant for comparison with SolTerm [2] are signalled in red. Only hot water preparation is relevant for this purpose. The input, Esolin, for purposes of comparison, was determined by the SolTerm data base.

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Fig. 1- Heat Balance for solar pre-heat system/solar-only system (according to EN 15316-4-3). The terms

relevant for the present work are signalled in red.

 

In this short description, the nomenclature of the standard will be used. In some situations, clarification of the terms is need since they do not correspond to the common solar thermal vocabulary according to EN ISO 9488 [4]. This is the case of the term “auxiliary”, which the standard uses for the electrical energy for pumps and controllers and for which the Solar Energy Vocabulary (EN ISO 9488) [4] uses the term “parasitic energy”.

The main objective of the standard EN 15326-4-3 [3] is the calculation QHW, sol, out, i. e., the total heat delivered by the thermal solar system to space heating and domestic hot water distribution system. As already stated, the present discussion will be centred on the calculation of QW, sol, out . Two methods are considered for this calculation:

– Method A uses the results of tests performed to Factory Made Systems according to EN 12976-2

[5] .

– Method B uses as input data the solar thermal system component data, i. e. system dimensional characteristics and data from component test and state of the art knowledge, such as:

Collector parameters: collector aperture area, zero-loss efficiency, heat loss coefficients;

Collector loop thermal losses and thermal losses of the distribution between storage tank and the distribution system (length of pipes, insulation and efficiency);

Climatic conditions: solar irradiation, outdoor air temperature;

Thermal losses of the solar storage tank

Storage tank parameters: type of storage tank, size;

In this paper all comparisons are performed considering the method B, based on the f-chart method

[6] . The general calculation of solar output is given by the expression:

=(a + bX + cY2 + dX2 + e¥3)-о, Лш„ [KWh] (1)

Where Qsolusm is the monthly heat use applied to the thermal solar system [kWh], usually termed

as heat demand. a, b, c, d, e are correlation factors depending on system configuration[8] and X and Y are dimensionless factors.

The term Qsohout, m represents the monthly values of QW solout For the determination of X the following equation is used:

X = A ■ Uloop ■ Л loop ■ AT ■ fst ■ tm /(Qsol, us, m ‘ 1000) (2)

In this expression of X, the term AT is a reference temperature difference calculated according to:

AT = Є„, – Q. ae [K] (3)

The heat loss coefficient of the collector loop, i. e., collectors and pipes, is determined by the collector characteristics and the insulation of the pipes. The heat loss loop coefficient is calculated by:

Uio0p = a1 + a ■ 40 + Ul0op, p / A [W/(m2.K)] (4)

The value Y is calculated according to eq. 5. It depends on the collector data (zero-loss collector efficiency) and the solar irradiance on the collector plane:

Y = A ■ IAM ■ Ло • Aioop ■ Im ■ tm /feoum •1000)

The term Qsoi, us, m is calculated considering the daily load volume, a fixed load temperature and a fixed cold water temperature:

Qsol, us, m Vload’P’Cp'(Tload (6) with p=1kg/litre and Cp = 4187 kJ/kg°C.

For some of the above parameters, the standard suggests default values. The values considered in the comparison were chosen either in accordance with the default values of EN 15316-4-3 or with those considered in SolTerm [2]. For determination of the thermal solar system output, care has to be taken in order to consider only Qsol, out, m < Qsol, use, m and Qsol, out, m > 0.

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