The theoretical analysis will be based on the average data recorded at the Tartu-Toravere Meteorological Station (TOR) of the Estonian Meteorological and Hydrological Institute [4] (http://www. emhi. ee/index. php? ide=8,74). This station is located in the south-eastern part of Estonia (58.25° N, 26.5° E, 70 m a. s.l.). It is a high-quality radiation monitoring station in the Baseline Surface Radiation Network. Radiation data, measured continuously since 1955, characterize the whole region from Scotland up to the north-west Russia. The modest actinometrical resource at TOR ~980 kWh-1m-2 is prevailingly concentrated on the summer season and can be converted (to electricity or heat) only partially due to the selectivity of solar collectors on azimuth. The analysis will cover six summer months from April to September, when radiation is technologically useable. We will ignore the efficiency of energy conversion, but will consider the incident angle modifier k0 for the beam component. Incident angle modifier [5] is a continuous variable that depends on the incident (attack) angle of the beam component 0.

k9 = 1 — tan1/r (<9/2), r є {0.25 — 0.4}.

Also, the effective (convertible) beam irradiance is the product of the “natural” (actinometrical measured) irradiance and the said incident angle 9 modifier. The diffuse component will be approximated with its isotropic model. It means that the performance of the module due to the diffuse component does not depend on its direction, but on the volume of the “visible” sky sphere, characterized by the angle ^x in Fig. 1. This is a simplification that involves some error. Declination 5 has the step of one month. Clock angle ю has the step of 10 minutes to be matched with the recording interval at the experiment1 to study the transient processes. Accordingly, the original hourly dataset [4] was interpolated into a 10-minute dataset.

The location and time variables assess the zenith angle 9z [6] and together with the construction variables the current incident angle 9. In our examples, construction variables are: the initial tilt angle (mostly p0 = 45°), initial azimuth (y0 = 0°), and the deflection angle x =var. Incident angle modifier k9 (yr) depends on the relative azimuth of the sun yr, i. e. the difference between the azimuth of the sun as and the azimuth of the projection of the gradient line of the deflected module on the horizontal plane. GbT is the calculated beam irradiance on the deflected module to be corrected due to the incident angle modifier.

A tilted module can “see” only a part of the half-sphere of the sky, limited due to the tilt angle of the deflected module Px. In the farm its visible sector is additionally limited because of the neighboring module shading effect. Therefore, the corrected diffuse component Gd has to be

The experiment was made at Tallinn University of Technology 59.4° N, 23.7° E, 30 m a. s.l.

limited proportional to the “visible” sector yx Fig. 1. The sum of both of the corrected (beam and diffuse) component values on the deflected module is the effective global irradiance GTx, which is converted into electricity (and heat). A flow-chart of the simulation model is shown in Fig. 2. Due to predicted volume of the present paper we cannot discuss some practical problems, which involve some additional limitations.

The CCT test method, developed by the SPF in Switzerland [4] is also an indoor test method and is similar to the DC test method, except that the test sequence of the CCT uses a longer test cycle and has a duration of 12 days instead of 8 days plus 18 hours preconditioning phase. The 12 days of the core phase are chosen with respect to annual seasonal average values of solar irradiation and ambient temperature.

Another important difference is that the building is simulated on-line so that the heat supply is controlled via the controller of the solar combisystem and therefore provides the right flow rates and temperatures. This ensures that all system functions may be assessed, which is one major advantage of this kind of test method. In contrast, the disadvantage is that the systems supply the emulated building with varying amounts of energy and there is no uniform or predictable energy use for space heating. This complicates the characterization of the systems energy-related performance. Unlike the DC method, the CCT can be used in principle to characterize solar combisystems where the system intentionally uses the building’s thermal mass to optimize its heat storage strategy.

Nevertheless, the test method allows a direct comparison of system performance.

The performance of the dry cooler and the cooling tower in the three sites are here evaluated from an economic point of view. The actual interest is to evaluate the effective costs of the heat rejection equipment per kWh of cooling energy produced. This is defined as the ratio between the costs of the heat rejection technology and the cooling energy delivered by the chiller in 20 years of operation. The costs include the initial investment to purchase the equipment (cooling tower or dry cooler) and the operation costs (for electricity and water) in 20 years. Relating the costs directly to the final cooling energy obtained by the system, allows to take into account how the different heat rejection technologies affect the absorption chiller capacity and COP.

The equation 2 defines the previous index.

The results are reported in the following Table 3.

2. Conclusions

In the present work a deck for dynamic simulations in TRNSYS of a small Solar combi+ system has been presented where an EES based code for the dry cooler was integrated. Although the water consumption for evaporative cooling was taken into account, from this first analysis it was observed that cooling tower technology has a much lower effective cost than dry cooler in all the three different locations. However this result has still to be investigated more in details, developing a more detailed model for the cooling tower and spending more efforts in fan control strategies. Different manufacturer data for both the technologies will also be considered, as in the present study the cooling tower had a much lower electrical consumption and still a very good performance for air entering in quasi saturated conditions compared to the dry cooler. Finally other considerations like local legislation on water make-up consumption and legionella disease concerns might be deciding factors for the deployment of dry cooling technologies.

Acknowledgements

The authors would like to gratefully acknowledge the financial support of the STIFTUNG SUDTIROLER SPARKASSE.

References

[1] W. Weiss, Solar Heating Systems for Houses, a design handbook for solar combisystem, Task 26, IEA.

[2] G. Franchini et al, Renewable cooling with solar assisted absorption chiller: system design, 62° Congresso ATI, Salerno, Italy, September 11-14, 2007.

[3] . F. Besana et al “Heat rejection technologies for solar combi plus system”, 2nd International Conference Solar Air-Conditioning.

[4] G. Dierks et al, Technical and Economic Evaluation Of Air-Cooled Cooling Systems Refrigeration And Air Conditioning Technologies, Jaggi/Guentner Report.

[5] D. G. Kroger, Air-Cooled Heat Exchangers and Cooling Towers, PennWell Corp., Tulsa, Oklahoma 2004.

[6] S. A. Klein et al, 1996, “TRNSYS — A transient system simulation program” Solar Energy Laboratory, Univ. of Wisconsis, Madison, USA.

[7] G. Nurzia et al, Combined Solar Heating and Cooling Systems: Simulation and Design Optimization, ASME International Solar Energy Division 2008, August 10-14, 2008, Jacksonville, FL, USA.

[8] W. M. Kays and A. L. London, Compact Heat Exchanger, Third Ed., McGraw-Hill Book., New York.

[9] J. E. Gonzalez, L. H. Alva, Simulation of an Air-Cooled Solar-Assisted Absorption Air Conditioning System, Transaction of the ASME, 276/Vol. 124, August 2002.

[10] ASHRAE Equipment Guide, American Society of Heating, Refrigerating, and Air Conditioning Engineers, Atlanta, 1983.

[11] J. C. Kloppers et al, A critical investigation into the heat and mass transfer analysis of counterflow wet­cooling towers, International Journal of Heat and Mass Transfer 48 (2005) 765-777.

[12] B. Cohen, Variable Frequency Drives: Operation and Application with Evaporative Cooling Equipment, CTI Journal, Vol. 28, No. 2.

[13] H.-M. Henning, Solar-Assisted Air-Conditioning in Buildings, A Handbook for Planners, SpringerWienNewYork.

J. Metzger*, T. Matuska and B. Sourek

CTU in Prague, Dept. of Environmental Engineering, Faculty of Mechanical Engineering, Technicka 4, 166 07 Prague 6, Czech Republic

* Corresponding Author, iuliane. metzger@fs. cvut. cz

Abstract

Energetic behaviour of solar combisystems equipped by solar flat-plate collectors with different grade of evacuation in various collector-envelope configurations operating to different types of heating systems has been investigated through computer simulations. Performance of given solar combisystem configurations and influence of collectors on the building performance (winter gains and summer loads) have been investigated. The simulation results underline the high performance of solar collectors with low heat loss and advantageous direct integration of solar collectors into the building faqade compared to separate installations (higher solar fractions, lower stagnation time). Solar collectors thermally coupled with low-energy building envelope do not affect the indoor environment dramatically when applied in usual design parameters even for high-performance vacuum collector case.

Keywords: solar collector, solar combisystem, building integration, evacuated collector

1. Introduction

Progressing tendency in low-energy and passive housing has increased the demand for solar combisystems for domestic hot water preparation (DHW) and space heating (SH). Following the European strategy [1] which now fully promotes the solar thermal heating and cooling sector and aims towards the standard of a ‘Solar Active House’ (heating and cooling demand is covered to 100 % by solar thermal energy) high potentials for solar combisystems are offered to meet this demand. Additionally, together with low-energy housing, an interest for solar collector integration into the building envelope has arisen to meet not only the technical advantages (lower heat loss of collectors, passive heat gains in winter) but even aesthetical and architectural demands. Moreover, the integration of solar collectors into the building envelope instead of separate installation represents transition from the concept of envelope considered as a heat loss to advanced multifunction envelope being both building construction and source of renewable heat (energy active envelope). Constructional building integration of solar collectors lying in the replacement of building envelope construction by the solar collector seems to be a challenging issue crucible for future development and spreading of solar technologies.

In preceding studies [2-4] solar DHW systems and combisystems with envelope integrated flat- plate collectors have been investigated from the point of system performance (solar gains, solar fraction, stagnation level) and building behaviour (winter heat gains, summer loads). Extensive parametric simulation analysis of solar combisystems presented in the paper has been performed to take solar flat-plate collectors with different grade of evacuation in various collector-envelope configurations operating to different types of heating systems into consideration.

2. Model description

Parametric analysis has targeted the energetic behaviour of solar combisystems for domestic hot water and space heating in a low-energy house with three types of heating systems (air heating, radiators, radiant heating) and three levels of pressure inside solar flat-plate collectors (atmospheric, subatmospheric evacuated to 1 kPa and vacuum with 1 Pa) and different types of collector installation (separate installation at slope 45°, integration into 45°sloped roof and integration into vertical facade).

A simplified tool based on the Energy Performance of Buildings Directive algorithms will be elaborated. As this simplified tool will be mainly dedicated to installers, a special attention will be paid to the easy to use, and the ergonomic.

At the end of the project, the design guidelines for manufactures based on the different work package results will be provided. This document will then be presented to manufacturers thanks to the dissemination activities.

As a result of this project, some recommendations for the subsidy scheme to national and local authorities will be produced, and the energy savings potential of solar combisystems in Europe will be evaluated.

One possible configuration of a combined solar and wood pellet system is shown in Figure 1, and other common solutions can be found in [10]. The systems can either be integrated, with the pellet burner integrated in the storage tank, as in the figure, or consist of separate sub-systems connected with a joint accumulator tank. Technically, the solar and wood pellet systems complement each other well. The solar collectors can supply all heat during summer using an electrical back-up heater. The pellet boiler, which usually operates with a lower efficiency and many starts and stops during summer, can then be completely turned off all season. Combined systems thereby result in an increased system efficiency. Both heating systems are additionally regarded sustainable and relatively cost-efficient, and for Swedish conditions, where large areas of forest are grown, biomass is a vast domestic energy resource. Biomass is however considered a limited resource, and by introduction of 10 m2 solar collectors in a pellet heating system, the pellet consumption can be reduced by 25 % according to [6].

3. Results

In the following, interview results for four topics are compiled; the present situation and obstacles, means to increase the use, costumers in the purchase phase and how to reach success. All statements in this section are from the informants and quotations have been translated to English by the author.

In the case of azimuthal tracking systems, rotation around the vertical axis (y orientation angle) is not relevant for the loading of the structure. The geometry of the structure is mainly changed from the a orientation angle. The extreme positions of this movement must impose the loading cases that must be considered:

• Loading case 1: a = 67°, corresponding to summer solstice at noon;

• Loading case 2: a = 9°, corresponding to winter solstice at sunrise.

The set point temperature of the auxiliary heater in both bikini and tank-in-tank solar combisystems is determined in such a way that the solar heating system can fully cover the space heating load and the DHW load. Table 1 shows the set point temperature needed for the tank-in-tank solar combisystem for three different houses and the Task 26 DHW profile. It can be seen that Model 6 and 7 have low set point temperature due to the large auxiliary volume of the hot water tank.

Table 1. Set point temperature of tank-in-tank system for three houses with the Task 26 DHW profiles

tank-in-tank solar

combisystem for three different houses with high, medium and low flow DHW profiles are shown. For the tank-in-tank system, the set point temperature is the same for the houses with low and medium heat demand and higher for the old house with a high space heating demand.

Table 2. Set point temperature of bikini solar combisystem and tank-in-tank solar combisystem for three
different houses and for high, medium and low flow rates DHW profiles for all seven models

In the theoretical analysis we suggest that energy of the beam radiation is converted proportional to the illuminated area. That is an idealization. Instead of the changing illuminated area, we can image the constant area and changing proportional to the illuminated area, the instant value of the irradiance G(ra). It means that we consider GTx (ю) being modulated by the position of the sun in the interval raF <ю <raG. From the point-of-view of energy generation, the results are equal. It means that the instant value of the irradiance is additionally modulated (multiplied with the relative share of the illuminated area of the module).

E = J Ac (ю) • GTz(a) = J Ac • GЧ(ю). (3)

Here the symbol GTx*(ra) means the modulated value of the irradiance on the tilted plane (of the deflected module) GTx*(ra) = GTx(ra)-AC(ra)/AC.

Equation (3) shows the theoretical energy produced during the transient shading in the morning.

The process in the evening is equal, but it develops vice versa.

From the 2-D model of the farm we can find

£ =a tan(1/((DR/WC) /(sin(x)-1)) (4)

or otherwise

t= Sin x. (5)

dR — cosx

We can see that the transient shading process (defined in angle units) is a function of two parameters depending on the deflection angle x and the density of the module columns in the row.

To calculate the energy we have to find the moment (solar hour raF) when the illumination starts from the upper edge of the module. The result of the simulation is shown in Fig. 3 for a south­faced farm. The moment of the start depends on the month (due to changing declination 5). It is independent of the latitude. While the shadow is moving over the module, the speed of its movement is important. Growth of the illuminated area of the module depends on the solar azimuth as. Figure 4 shows that in the morning and in the evening the speed of the changes of the solar azimuth (its derivative) is practically constant and nearly independent of the month (declination). While p0= 45°, it has the value das /dra »“0.85”«const and for a simplification may be considered as “1”. This quality allows us to do a rough analysis by substituting the change of the solar azimuth angle for the change of the clock angle. Therefore, the illuminated area AC (ю) is proportional to the current clock angle, i. e. it is increasing (and decreasing) linearly. The duration of the calculated transient shading for an inner module is shown in Fig. 5.

The objective of this work is to further develop the Concise Cycle Test Method (CCT).

3.1 The different phases of the test sequence

As the CCT method, the test procedure is divided into five parts. The first three phases are for preconditioning purposes that bring the storage tank to the right energy level for starting the core phase. This core phase contains a climate and DHW load sequence of twelve days that nearly corresponds to annual reference conditions. The core phase is followed by a final discharge phase which is one day long.

The difference between the CCT and the INES method lays in the methodology of the choice of days.

The core phase contains a sequence of twelve days that are selected from an annual weather data file for different locations. In previous work, the core phase days contains realistic climate and load conditions. Several specific days were chosen for each ‘season’ during one test sequence, e. g. four days for winter conditions, four days for summer conditions and four days contain spring/autumn conditions. Mean values of temperature/irradiation and load variables (domestic hot water draw offs and space heating) of each ’season’ correspond to average values for the seasons of the whole year. All days together represent the average weather and load conditions of a whole year.