3.1. Short descriptions

The Portuguese legislation transposing the EU Directive 2002/91/CE, includes a Solar Thermal Obligation, imposing the usage of solar thermal collectors for hot water preparation if there are favourable conditions for exposure (if the roof or cover runs between SE and SW without significant obstructions) in a base of 1m2 per person. The energy necessary for the preparation of hot water constitutes one of the terms for evaluation of energy performance of the building, as well as, the heating and cooling loads of the building, according to:

This term is calculated per apartment area, Ap, and the term Qa is the heat demand for hot water preparation given by in equation (8) and it is equivalent to Qsol, use. pa is the efficiency of the conventional heating equipment used for hot water preparation (backup system). The term Eren is any other renewable energy that is used for hot water preparation or that substitutes the thermal solar system, according to the permitted cases in the legislation.

Qa =(MAQS • 4187-AT • nd )/(3600000)[kWh/yr] (8)

The mass of water to be heated is equivalent to a volume of 40 l per conventional occupant. Also the collector area to be installed is a function of the number of conventional occupants, considering 1 m2 per occupant. Conventional occupants are a function of the home typology, i. e., the number of rooms, according to Table 1.

In equation 8 the value of AT is equal to 45°C, i. e., Tload — Load temperature — 60°C and Tcold — Temperature of cold water (mains water) — 15°C.

The value nd is the number of days where hot water preparation is needed. In the case of residential buildings is equal 365.

Tablel — Number of conventional occupants in an appartment Apartment typology T0, T1 T2 T3 T4 Tn N. of conventional occupants 2 3 4 5 N+1

The term Esoiar is calculated using the software tool developed by INETI and called SolTerm [2], whose characteristics were shortly described in section 2.2.. This term is equivalent to QW, sol, out.

2. Results

For comparison of the two methodologies default values introduced in section 2. were used. The comparison is based on the value Qsol, out, m.

Calculations were made for one location only, Lisbon, and the monthly values of Esol, in were derived from SolTerm data base. Two flat plate collectors, one selective and one non-selective, were considered. In Table 2, the parameters of the two collectors are listed.

Table 2 — Characteristic parameters of the collectors used for comparison purposes. Collector A [m[9]] % [-] a1 [W/K m2] a2 [W/K2 m2] IAM [-] A(selective flat plate) 2.3 0.77 3.5 0.017 0.93 B(non-selective flat-plate) 1.65 0.66 5.9 0.039 0.95

Calculations of Qsol, out were made and the yearly values were compared for different system configurations considering that the system load volume varied according to the apartment typologies given in Table 1. Results are shown in Table 3.

Table 3 — Energy delivered by the solar thermal system for hot water preparation. Qsol out vear [KWh] Typology/ mandatory collector 2 area 2 A Vload Vstore [liter] EN 15316 SolTerm 5.0 (%) Collector A T2 (3 m2) 2 col x 2.3 m2 120 1972 1970 -1.4 T3 (4 m2) 2 col x 2.3 m2 160 2453 2492 0.9 T4 (5 m2) 3 col x 2.3 m2 200 3271 3261 -2.0 T5 (6 m2) 3 col x 2.3 m2 240 3747 3772 -0.3 T6 (7 m2) 4 col x 2.3 m2 280 4571 4544 -2.3 Collector B T2 (3 m2) 2 col x 1.65m2 120 1245 1401 11.1 T3 (4 m2) 3 col x 1.65m2 160 1824 1992 8.4 T4 (5 m2) 3 col x 1.65 m2 200 2033 2274 10.6 T5 (6 m2) 4 col x 1.65m2 240 2598 2882 9.9 T6 (7 m2) 5 col x 1.65 m2 280 3162 3465 10.6

The comparison between the two methods shows a strong difference as a function of the collector type, i. e., its efficiency parameters. While for a selective collector, the results obtained with the two methods have differences lower then 2%, in the case of the non-selective collector, differences can be of the order of 10%, with underestimation by the methodology of the standard EN 15316-4­

3.

To explain this discrepancy, it is necessary to recall that the methodology of EN 15316-4-3 is based on f-chart method [6] and that this method is a result of correlations derived from several simulations using a specific configuration system and TRNSYS programme [8]. In reference [6] the range of design parameters is indicated. Two of them are dependent on collector efficiency parameters; 0.6 < (xa)n <0.9 and 2.1 < UL < 8.3 W/K m2. For flat plate collectors (xa)n =p0 and UL is the heat loss coefficient not dependent on temperature. In the case of the non-selective collector, using the efficiency curve with parameters a1 and a2 to determine a linear approximation (up to 0.07), a UL of 8.4 W/K m2 is obtained, showing that it is outside the limit of values considered in the correlations of the f-chart method.

Results considering different ratios of storage tank volume and collector area were also obtained as can be seen in Table 4 for the case of selective collector listed in Table 2.

Table 4 — Solar fraction (%) calculated for thermal solar systems with selective collectors. Vstore = 1000 l Vstore = 2000 l Vstore = 3000 l EN 15316 SolTerm 5.0 EN 15316 SolTerm 5.0 EN 15316 SolTerm 5.0 100 l/m2 42.2 43.1 46.9 47.9 45.7 46.8 75 l/m2 57.5 58.6 57.8 59.4 55.8 57.3 50 l/m2 73.8 74.9 72.0 73.6 72.9 74.7 25 l/m2 89.8 88.4 90.4 89.1 90.3 89.2

In this case, the comparison is presented by the value of yearly solar fraction. The differences using both methodologies are not dependent on the ratio between storage volume and collector area.

3. Conclusion

The European Standard EN 15316 (part 4-3) [3] includes a methodology for calculation of the energy delivered by a thermal solar system for hot water preparation based on the f-chart method

[6] . This methodology is easy to apply and can even be implemented in an Excel Sheet.

The in the work developed, a comparison of this calculation procedure with the methodology adopted in the Portuguese legislation [1] was presented, i. e., calculation using the SolTerm programme [2]. SolTerm 5.0 was the version used for comparison purposes. It is possible to see that the results obtained with both methodologies are comparable when the collectors used are of the type flat plate selective collectors.

If non selective flat plate collectors are considered the differences between the two methodologies can be of the order of 10%, where EN 15316-4-3 [3] corresponds to an underestimation of the energy delivered by the solar thermal system. The possible explanation for this difference is the fact that the methodology of EN 15316.part4-3 [3] is based on f-chart method [6], which has application limits dependent on the collector efficiency parameters.

Further investigation is necessary in the case where the collector efficiency is higher then the typical values for selective flat plate collectors, as is the case of evacuated tube collectors.

Possibility of adoption of the methodology of EN 15316 in the calculation of solar space heating systems is limited due to the fact that the standard only presents correlation coefficients for one type of space heating systems — direct floor heating system.

collector aperture area according to EN 12975-2 [m2]

heat loss coefficient of solar collector related to the aperture area according to EN 12975-2 [W/Km2] temperature dependent heat loss coef. related to the aperture area according to EN 12975-2[W/K2 m2]

Incident solar energy on the plane of the collector array [kWh/m2] storage tank capacity correction factor [-]

average solar irradiance on the collector plane during the considered period [W/m2]

incidence angle modifier of the collector = К50(та), from the collector test standard EN 12975-2 [-]

Heat delivered by the thermal solar system to domestic hot water distribution [kWh]

Heat delivered by the thermal solar system to space heating distribution system [kWh]

Total heat delivered by the thermal solar system to space heating and domestic hot water distribution systems [kWh]

Auxiliary electrical energy for pumps and controllers [kWh]

recoverable auxiliary electrical energy for pumps and controllers. Part of the auxiliary electrical energy, which is recoverable for space heating [kWh]

internally recovered auxiliary electrical energy for pumps and controllers. Part of the auxiliary electrical energy, which is transferred as useful heat to the thermal solar system [kWh]

non recoverable auxiliary electrical energy for pumps and controllers. Part of the auxiliary electrical energy, which is neither recoverable for space heating nor transferred as useful heat to the thermal solar system [kWh]

Total thermal losses from the solar system [kWh]

thermal losses from the thermal solar system, which are recoverable for space heating [kWh]

Non recoverable thermal losses from the thermal solar system. Part of the total thermal losses, which are not recoverable for space heating [kWh]

monthly heat use applied to the thermal solar system, usually termed as heat demand [kWh] temperature needed for hot water preparation [°С] temperature of cold water [°С] length of the month [h]

heat loss coefficient of the collector loop (collector and pipes) [W/(m2.K)]

Uoop p overall heat loss coefficient of all pipes in the collector loop, including pipes between collectors and array

pipes between collector array and solar storage tank [W/(m2.K)]

Vload Daily volume of hot water needed for hot water preparation [l]

n0 zero-loss collector efficiency factor obtained according to EN 12975-2 and related to the aperture area [-]

nloop efficiency factor of the collector loop taking into account influence of the heat exchanger [-]

0 re/ reference temperature depending on application and storage type [°С] average outside air temperature over the considered period [°С]

e, avg

p water density [kg/liter]

References

[1] RCCTE, Portuguese Thermal Performance Building Code (Decreto-Lei n.° 80/2006, DR 67 SERIE I-A, 2006-04-04),

[2] SolTerm, Version: 5.0.2 — 27th April 2007, (Authors: Ricardo Aguiar and Maria Joao Carvalho), CD — ROM distribution, ISBN 978-972-676-205-8

[3] EN 15316-Part 4-3 (2007), Heating systems in buildings. Method for calculation of system energy requirements and system efficiencies. Part 4-3: Heat generation systems, thermal solar systems.

[4] EN ISO 9488 (1999), Solar Energy — Vocabulary

[5] EN 12976 (2006), Thermal solar systems and components — Factory made systems — Part 2: Test methods, European Standard.

[6] J. A. Duffie and W. A. Beckman, Solar Engineering of thermal processes, John Wiley and Sons, 3rd edition, 2006, Chapter 20 — Design of Active systems: f-chart.

[7] EN 12975 (2006), Thermal solar systems and components — Solar collectors — Part 2: Test Methods, Section 6.1. European Standard.

[8] TRNSYS: A Transient System Simulation Program (Version 15), S. A. Klein, W. A. Beckman and P. I. Cooper, Solar Energy Laboratory, Madison Wisconsin, 1998.