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How large does a large-scale system need to be?

Small-scale solar systems providing hot water heating for private households are available as complete packages in many different sizes

and variants. Complete solutions for 3, 4 and 5 per-son households, using vacuum tube or flat plate collectors, are available. These systems are easy to design: the number of heads in the residential unit to be supplied is counted and an extra collector can also be added if the building budget allows this. The result is often also influenced by the available roof area or aesthetic considerations.

A sensible selection of the collector area required is not so easy in solar systems used for heating both hot water and the building itself. It is often not possi-ble to avoid excess heating capacity in summer, even when the backup heating contribution is relatively small in existing buildings with average thermal insu-lation. However, when in doubt it is still possible to

Individual planning of the necessary collector area is unavoidable when

building a large-scale solar system. Simulation programmes alleviate the

planner from the need to perform calculations, but an understanding of the

underlying principles is nevertheless important.

fall back to standard packages, rules of thumb and manufacturer’s recommendations.

The situation becomes more complicated when the solar system must supply an apartment building, hotel, hostel or similar building. This requires individ-ual planning. Despite the current availability of de-sign and simulation programmes for solar systems it is advantageous to understand and have a good com-mand of the mechanisms used for designing solar systems – also to allow critical examination and interpretation of the results provided by simulation calculations.

The parameters used in the calculation

The major parameters used for designing the collector surface are:

There are no universal dimensioning rules for large-scale collector arrays such as these. Photo: Martin Schnauss

Sun & Wind Energy 3/2012 55

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1) the solar irradiation at the installation site2) the total consumption (heating requirements for

hot water heating, circulation losses, building heating and other factors) and the consumption profile

3) the desired coverage ratio.

Where can you obtain this information?

The levels of solar irradiation are well known and suf-ficient meteorological data is available. Compared to the wind, this is subject to small annual fluctuations. In general, a high degree of precision is not required for the weather data. The consumption is usually the largest unknown factor when dimensioning the col-lector area. It is often incorrectly estimated – usually rather too high than too low. This results in incorrect dimensioning, low degrees of usage, excess heat pro-duction and frequent states of stagnation. The usual regulations used for determining hot water heating requirements (e.g. DIN 4708) are designed for dimen-sioning conventional systems that can ensure a full supply and which must also accommodate peak loads. A completely different procedure must be used for designing solar systems because these are usual-ly operated alongside the conventional systems in order to conserve fossil fuels. For example, the mini-mum consumption (low load) at times of high solar ir-radiation is an important criterion. Since this informa-tion can only really be obtained by actually measuring the consumption, this method should always be used wherever possible in order to provide planning secu-rity. To achieve this, e.g. a water meter is installed in the cold water supply of the hot water heater and the readings are regularly recorded. In addition to this, measures should also be taken to reduce the amount of consumption in the existing building (e.g. through the use of water-saving taps) in order to reduce the size of the required solar area from the very begin-ning. Foreseeable changes in water usage should also be taken into consideration in the planning process.

The ratio of solar coverage is a target specifica-tion for designing the collector area and specifies the proportion of the total heating requirements that is to be provided by the solar system. For hot water heat-ing systems this is usually 30 % to 60 %, whereby at values of 70 % upwards the costs increase dispropor-tionately to the usefulness in our latitudes. Addi tional factors to be considered are the collector technology to be used (flat plate collectors, vacuum tubes), the alignment, inclination and possible shadowing of the solar surface and system efficiency (dependent on the system technology, hydraulic issues, storage tank losses, layered storage factors, control system and working temperature of the collector).

These factors are taken into consideration by ap-propriately increasing or decreasing the size of the required collector area. Vacuum tubes and highly ef-ficient systems with well insulated layered storage

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Solar thermal dimenSioning

tanks can increase yields by about 10 % and the influence of alignment and shadowing can be determined using tables, diagrams and simulation calculations.

In addition to the solar fraction, other criteria for evaluating a solar system is the yield/specific

yield (kWh/m² a): the yield is the heat transferred to the hot water. The specific yield is the yield at the collector surface. Another criterion is the system utilisation ratio (%): the system utilisation ratio represents the relationship between the yield and the energy irradiated onto the collector surface. Investment costs have to be considered as well: the investment cost includes all costs for erection of the system whereas the specific investment cost is obtained by dividing this by the collector surface ar-ea. Moreover, the heating price (€/kWh) is impor-tant: it includes the investment costs, financing costs, operating costs, the yield and the service life of the system.

What characterises a “well designed” solar sys-tem? A high degree of solar coverage? A high specif-ic yield? Or a low-cost system that provides solar en-ergy at an economical heating price? Figure 3 illus-trates the problematic nature of designing solar sys-tems. This shows the annual course of hot water consumption (blue), the irradiation on collectors of

Collector surface area [m²]

Specific yield [kWh]

Annual solar yield [kWh] Solar fraction [%] System utilisation rate [%]

14 519 7,300 29 46

17 500 8,500 34 44

25 432 10,800 43 38

33 388 12,800 50 35

40 354 14,200 55 32

44 340 15,000 58 30

50 310 15,500 60 28

Figure 2: While the absolute annual yield and the solar fraction increase with collector area, system utilisation and specific yield are declining.

Abbildung 2 Die Parameter für die Auslegung von Solaranlagen verhalten sich teilweise gegenläufig. Um Anlagen zu optimieren gilt es, für die jeweilige Situation die richtige Balance zu finden.

Kollektorfläche

Deckungsanteil Investitionskosten

Wärmepreis Überschüsse

Kollektortemperatur

Verbrauch

Jahresnutzungs- grad

Spezifischer Ertrag Auslastung

Collector surface areaSolar fraction

Investment costsHeating price

Surplus productionCollector temperature

ConsumptionAnnual degree of

usageSpecific yield

Load

Figure 1: The parameters for designing solar sys-tems work in opposition to a certain degree. The correct balance for each respective situation must be found in order to optimise a system. Graphics (3): Martin Schnauss

Sun & Wind Energy 3/2012

three different sizes (shades of yellow) and the re-spective degrees of coverage (overlapping). The in-creasing surpluses resulting from increasing collector areas can be clearly seen. These reduce the annual usage efficiency and the specific yields per square metre of collector surface area.

The design parameters therefore work in opposi-tion to a certain degree. Attempting to optimise one parameter unavoidably worsens the others.

Increasing the collector surface area at constant consumption increases the degree of coverage and investment costs but also increases the surplus heat-ing and heating price. This reduces the degree of usage and specific yield.

If consumption increases with a constant collec-tor surface area, then the degree of usage and the specific yield in-crease but the degree of coverage, surplus and heating price de-crease. The opposite effects occur when, for example, the consump-tion was overestimated during planning and is less in practice than the calculated value. This can have fatal consequences for the economic viability of the system.

In this sense there is no “cor-rect” design but rather an optimum balance between various factors depending on the specific situa-tion and target specifications. The aim of the design process is there-fore to find the correct balance be-tween the different parameters.

One task – four solutions

The following different design pos-sibilities are explained based on a sample apartment house with 10 residential units and 30 persons located in Germany.

Solar irradiation: In Germany the solar irradiation is 0.5 kWh/m² on an average December day and 5 kWh/m² on an average day in July. The maximum solar irradia-tion striking an optimally inclined surface on a sunny summer day actually reaches a level of about 8 kWh/m² and this value is relatively independent of the site. The annual daily mean is about 3 kWh/m².

Consumption: An average consumption of 1,200 L/day at a temperature of 60 °C was meas-ured in the house. During the holiday period the consumption was only 1,000 L/day. The cold water temperature is 12°C.

To simplify matters, a solar system only for hot water heating is to be dimensioned. The energy required for hot water heating is calculated using the formula:

Q = m * c * ∆T (Q = quantity of heat [Wh]; m = mass [kg] (1 kg =

1 L for water), c = specific heat capacity (1.16 [kWh/kg K] for water) and ∆T = temperature difference be-tween cold water and hot water (48 K in this exam-ple))

For the annual average, this results in a value of: Q = 1,200 x 1.16 x 48 = 66,816 Wh = 66.8 kWh per day. In the holiday period, 55.6 kWh per day is required.

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Design A – Highest degree of usage: The specific yield can be optimised when the daily consumption is always greater than the daily solar yield. To this end, the system is therefore designed for the mini-mum level of consumption at the maximum level of solar irradiation – in this case the summer holiday period. Surplus heating therefore does not occur. Such systems are also called preheating systems because the water can only be preheated for most of the year. Low levels of solar coverage are accepted in this approach.

In our example, on a sunny day in summer a max-imum irradiation of 8 kWh/m² is available to supply a consumption of 55.6 kWh/day. With efficient systems a degree of usage of 50 % is realistic, resulting in a value of 4 kWh/day for each square metre of collector surface area. To cover the heating requirements the system would thus require 55.6 kWh/4 kWh/m² = 13.9 m² of collector surface area.Design B – Average consumption: If the holiday period is ignored and the average consumption of 66.8 kWh/d is used instead, then the calculation (also using a 50 % degree of usage) results in 66.8 kWh/4 kWh/m² = 16.7 m² of collector surface area.Design C – Maximum coverage in summer: If a maxi-mum level of solar coverage is to be achieved in sum-mer then, with a mean irradiation of 5 kWh/m² d in June and July and a degree of usage of 40 %, an aver-age of 2 kWh/m² per day can be provided. In this case, the degree of usage is dropped to a value of

A solar system on an apartment block stagnates more quickly in summer when most residents are on holiday at the same time. Photo: ESTIF

Sun & Wind Energy 3/2012 59

40 % based on experience with existing installations because higher temperatures and surpluses occur compared to example A. Compared to the second ex-ample, the collector surface area would double to a value of 66.8 kWh/d/2 kWh/m² d = 33.4 m².Design D – 60 % degree of coverage over the annual mean: If 60 % of the annual mean hot water require-ments must be covered, then 66.8 x 0.6 = 40 kWh per day must be provided. Due to surpluses and losses, only about 30 % of the annual mean irradiation of 3 kWh/m² d can be used by the collectors (a value gained from experience that can be checked via sim-ulation calculations), which means that the collectors provide 0.9 kWh/m². This means that a collector area of 40 kWh/0.9 kWh/m² d = 44.4 m² is required for this system.

Figure 3: The heat excess grows faster than the in-creased net energy when the collector area grows.

excesshot water requirement

increased net energy

Mar MayJan Feb Apr Jun Jul Aug Sep Okt Nov Dez

120

100

80

60

40

20

0

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Solar thermal dimenSioning

Such high degrees of coverage (> 60 %) are main-ly aimed for in small systems so that the boiler can be switched off outside the heating period. In larger sys-tems the boiler is usually not switched off and a high degree of coverage does not provide this effect.

Comparison with the simulation result

To now obtain the exact and complete characteristic values for the sample systems, these are simulated using an appropriate programme – in this case T*SOL. The following settings are used: standard flat plate collector, irradiation in Berlin, inclination 45° aligned to the south, storage tank volume of 60 L/m² of col-lector surface area. The illustrated results are now compared with the results from the simulation calcu-lations.Design A: With a collector surface area of 14 m² the simulation yields a coverage of only 29 %, with a re-spectable degree of usage of 46 %. The specific yield reaches an impressive value of 520 kWh. The total yield is 7,300 kWh.Design B: A collector surface area of 17 m² (20 % more than design A) produces a yield of 8,500 kWh, which is about 16 % more than design A. The degree of coverage increases by 5 % to 34 %.Design C: A collector surface area of 33 m² provides coverage of 50 % at a 35 % degree of usage. The spe-cific yield drops to 388 kWh (75 %). The total yield in-creases by 75 % to 12,800 kWh but this requires more

Sun & Wind Energy 3/2012

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than twice the collector surface area than design A.Design D: A collector surface area of 44 m² results in a 58 % degree of coverage, which is only slightly less than the planned value of 60 %. The simulation pro-gramme shows a 30 % degree of usage with a specif-ic yield of 340 kWh/m² per year. A collector surface area somewhat more than three times larger than design A approximately doubles the yield to 15,000 kWh/year.

These examples show the very wide spectrum of design variants that can exist, with collector surface areas differing by a factor of up to 3. Every design ap-proach is valid depending on particular design condi-tions. Those aiming for independence through the largest possible savings of fossil fuels and a high re-duction of CO2 emissions, or who are planning over the long term, should choose large collector surface areas and coverage degrees of 50 % and more. The amortisation times are shortened by high specific yields at low degrees of solar coverage.

Assigning prices to the different variants, where-by the specific costs depend on the size of the sys-tem, usually yields an optimum solution (usually somewhere in the middle of the range of systems considered). The economic aspect is thus often a de-cisive factor in tipping the scales towards a particular design decision. Martin Schnauss

Reference:VDI 6002 Solar heating for domestic water - General principles, system technology and use in residential buildings

With big solar systems like this one in Austria, you have to choose between a high solar fraction and a low price for heat.

Photo: ESTIF/Austria Solar