Active solar heating Water

Gerhard Faninger

12.2.1 Concept

Heating DHW with solar energy in a high-performance house is sensible. In such houses, the energy needed to heat domestic water can equal or even exceed the energy needed for space heating since the latter has been so far reduced by insulation and heat recovery. Furthermore, demand for heating domestic water is a 12-month energy demand, including the high insolation summer months. Using a solar system is therefore an effective way of reducing the total primary energy demand. Increasingly, the market for solar water systems also includes systems that provide, in addition to water heating, space heating in winter.

12.2.2 Components

Collectors

For a high-performance house, a high-performance collector is a good choice, particularly if the collector should provide both water and space heating. A high-performance collector is able to produce more heat during the shortened mid-winter heating season of such houses.

Flat-plate collectors. A high-performance flat-plate collector is characterized by a superior absorber and glazing. The absorber should have a coating with a high solar absorption, black paint (>95 per cent) and low heat emissivity selective coating (<5 per cent). The glazing should be anti-reflection treated and consist of a low iron glass type to maximize solar radiation transmitted to the absorber. Such flat-plate collectors can easily achieve outlet temperatures of 80°C with a conversion efficiency of about 50 per cent to 60 per cent.

Evacuated tube collectors. This type of collector achieves superior performance because the vacuum surrounding the absorber drastically cuts heat losses to the ambient. Outlet temperatures above 100°C are easily achieved with a higher conversion efficiency compared with a flat-plate collector. The inside-facing underside of the glass pipe has a reflective coating to irradiate the absorber from beneath. Thus, vacuum collectors have the further advantage of not requiring any given slope for optimal performance. The glass pipes can simply be rotated to the optimal incident angle for the application. For this reason, they can be mounted on a south façade or roof.

Properties of collectors. Collectors can be characterized by means of two experimentally determined constants:

1 Conversion factor: the collector efficiency when the ambient air temperature equals the collector temperature.

2 Heat loss coefficient: the mean heat loss of the collector per aperture area for a measured temperature difference between the collector and the ambient air temperature in W/m2K.

Figure 12.2.1 Efficiencies of different collector types under different conditions and appropriate uses, where Tk = collector working temperature (°C); Tu = ambient temperature (°C);

and G = solar irradiation (W/m2)

Source: G. Faninger, University of Klagenfurt

Figure 12.2.1 Efficiencies of different collector types under different conditions and appropriate uses, where Tk = collector working temperature (°C); Tu = ambient temperature (°C);

and G = solar irradiation (W/m2)

These collector constants are determined under exactly defined conditions (global radiation intensity, angle of incidence, air temperature, wind velocity, etc). The performance of different collector types and applications are shown in Figure 12.2.1. The efficiencies are given for the temperature difference between the collector and ambient divided by the solar radiation. Logically, as the collector gets hotter, the efficiency falls off. For heating of high-performance houses, selective coated collectors or vacuum pipe collectors are a good choice.

It is beneficial to integrate solar collectors within the building envelope for aesthetic and economical reasons. For roof installations, where the systems deliver heat over the whole year, the optimal tilt angle (northern hemisphere) is between 30° and 75°. The orientation can be between 30° east and 45° west. Figures 12.2.2(a) and 12.2.2(b) show calculated results for Stockholm, Zurich and Milan. As can be seen, a 90° tilt, or fagade-integrated collector, is far from optimum in all locations. However, it

Solar Irradiation on Tilted Surface, Influence of collector orientation

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Collector inclination: 45° South: 0°, West/East: 90°

Collector orientation, degree

Source: G. Faninger, University of Klagenfurt

Figure 12.2.2 (above) Solar radiation for different orientations in the reference climates (south = 0°, north = 180°); (below) solar radiation for different tilt angles in the reference climates for a south-facing collector

performs better in winter with low sun angles. When there is snow cover on the ground, it receives an extra portion of ground-reflected solar radiation. Roof collectors with too little slope, by contrast, will have zero output when covered by snow. A big limitation of façade collectors is, however, that by low sun angles, neighbouring buildings and trees will cast shadows on the collector surface. Façade collectors perform worse in Zurich than in either Milan or Stockholm. This is possibly explained by the higher frequency of overcast weather in Zurich.

Storage

Surprisingly, within a large range, the size of the tank relative to the collector area is not a major factor affecting system performance. This is evident in the example of an apartment block shown in Figure 12.2.3. Doubling the tank size increases the solar share by less than 15 per cent. More important is avoiding the mixing of the hot water at the top of the tank with cooler water at the bottom, the insulation of the tank and the avoidance of thermal bridges (for example, the feet of the tank). The size of the tank should not be oversized because that would increase heat losses and investment costs. For economy and energy efficiency, the tank should be from 1.5 to 2.0 times greater than the daily hot water demand, and the collector area between 1 and 2 m2/occupant. Typically, designs for solar hot water systems in households are 3 m2/300 litres for up to 3 persons, and 8 m2/500 litres for 4 to 5 individuals. Performance for these two variations is shown in Figure 12.2.4.

Source: G. Faninger, University of Klagenfurt

Figure 12.2.3 Influence of storage size on system output for 96 m2 of collectors serving an apartment block

Source: G. Faninger, University of Klagenfurt

Figure 12.2.3 Influence of storage size on system output for 96 m2 of collectors serving an apartment block

For solar-combined space heating and domestic water heating systems, the storage can be combined with the heat distribution subsystem - for instance, floor heating, which further increases the heat storage capacity. In this case, heat can be extracted from the lower section of the tank for the radiant floor heating and the upper part of the tank for DHW. The water supplied to the collector can come from a heat exchanger at the bottom of the tank where the coolest temperatures occur. This maximizes the collector efficiency. The location of the hot return from the collector to the tank depends upon the flow rate. For high-flow collectors, this connection can be quite low. On the other hand, this connection should be a higher or even better variable (stratified) for low-flow collectors.

For solar combi-systems where hot water is supplied to a water-to-air heat exchanger in the air supply duct or to a water-to-water heat exchanger in a water heat distribution system, the connection should be at the top of the tank, where the highest temperatures occur. For radiant floor heating, the connection can be in mid tank, since the required temperature supply is much lower.

12.2.3 Solar hot water preparation

The annual system efficiency of solar hot water systems ranges from 30 per cent to 70 per cent in a cold climate and can be even higher for milder climate types. Systems should be designed to cover nearly 100 per cent of the summer demand, as shown in Figure 12.2.5 for three climates. In temper-

Source: G. Faninger, University of Klagenfurt

Figure 12.2.4 Two schemes for sizing the storage-to-collector area for the three reference climates

Source: G. Faninger, University of Klagenfurt

Figure 12.2.4 Two schemes for sizing the storage-to-collector area for the three reference climates

kwhTmoenth Solar Hot Water System Solar Heat output

Solar System for Household

Collector area: 8 m (selective flat plate) Storage volume: 500 litre

120 litre/day (50°C)

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Annual neat output:

Stockholm: 2370 kWh/a Zurich: 2569 kWh/a Milan: 2753 kWh/a

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4

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Source: G. Faninger, University of Klagenfurt

Figure 12.2.5 Monthly solar heat output of a household solar DHW system with 45° tilt and 8 m2 of collectors supplying a 500 litre storage tank in three climates (demand 120 litres/day at 50°C)

ate climates, the annual coverage should reach 70 per cent for a single family house and 40 per cent for an apartment block. Coverage for the latter is generally lower because of lack of roof space for the stack of apartments below. Summer coverage in the temperate and mild climates should be designed to be under 100 per cent to reduce system overheating.

Solar hot water systems in an apartment house, although the coverage may be smaller, generally costs less per residential unit compared to systems for single family houses. Indeed, as the collector

800 700 600 500

UJ 400

300 200 100

800 700 600 500

UJ 400

300 200 100

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— ' Maximal costs -Arverage costs

- - Minimal costs

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Maximal costs:

installation on flat roofs Minimal costs: building integrated

150 200 250 Collector area, ms

150 200 250 Collector area, ms

Source: G. Faninger, University of Klagenfurt

Figure 12.2.6 Collector costs in relation to collector area

Source: G. Faninger, University of Klagenfurt

Figure 12.2.7 Solar heating kWh costs in relation to collector area

Source: G. Faninger, University of Klagenfurt

Figure 12.2.7 Solar heating kWh costs in relation to collector area area for a single installation increases, the unit costs decrease sharply up to approximately 100 m2, as can be seen in Figure 12.2.6. This translates to lower kWh costs for the solar heat, as illustrated in the example for Vienna in Figure 12.2.7.

Hygienic aspects of solar water heating. In some solar active system configurations the storage tank contains drinking water. There is then the risk of the so-called Legionnaire's disease, Pneumonia Legionella. It is caused by Legionella, or rod-shaped, mobile, aerobic bacteria that occur naturally in surface water and groundwater. They begin to propagate at temperatures between 20°C and 50°C, with optimum growth occurring between 30°C and 40°C. Above 60°C, they die off quickly. A long residence time in water at favourable temperatures may result in high concentrations of Legionella. Stagnant water in pipes or parts of an installation that have not been flushed are breeding grounds for these bacteria.

To prevent Legionella, the water temperature should either be below 25°C or above 50°C. Disinfecting a contaminated system can be done by flushing it, then heating the water to 60°C for 20 minutes. In general, solar thermal systems for hot water preparation are backed up by an auxiliary heating system to achieve temperatures above 50°C. In this manner the risk of Legionella contaminated water can be minimized.

A distinction is made between small and large systems:

• Small systems are considered to have a very low risk and do not need special attention. Small systems are installations in one or two family houses, or installation with a volume less than 400 litres and with less than 3 litres in pipes between the heater outlet and draw-off point.

• Large systems should be designed so that they can be heated up to 60°C to a frequency prescribed by the building/sanitary code.

12.2.4 Solar combined heating and hot water systems

Many configurations of solar combi-systems and backup systems have been tried. From 1975 to 1985, these systems were custom engineered. Through a cooperation of the International Energy Agency (IEA) Solar Heating and Cooling Programme (SHC) Task 26: Solar Combi-systems (see www.iea-shc.org/task26/index.html), existing designs have been analysed and optimized and a valuable design handbook produced (Weiss, 2003). Today, solar companies offering simple and economical systems frequently offer system design as part of their service. Components of solar combi-systems are factory assembled in compact units, making the onsite work of the installer easier and ensuring better reliability.

Currently, installed systems demonstrate that solar space heating is possible even in northern locations. In high latitudes, it would seem that such systems make little sense, given the minimal to zero solar radiation in mid winter. However, solar irradiation from September to October and March to May can make a useful contribution during the beginning and end of the heating season. Especially in northern countries, as well as in alpine areas, some heating during the summer period will increase living comfort.

In summer, the solar radiation is double that of winter, ensuring a high coverage of DHW demand. To achieve maximum overall system efficiency, priority is given to the load (DHW or space heating) with the lowest temperature level so that the solar collector works with the highest efficiency. Figure 12.2.8 shows the computed monthly performance of a combi-system in the Zurich reference temperate climate.

Source: G. Faninger, University of Klagenfurt

Figure 12.2.8 Solar share from a combi-system in Zurich for a detached house with 16 m2 of collector tilted at 45° and supplying a 2000 litre storage tank

12.2.5 Centralized solar heating systems

In centralized concepts, a central storage tank is supplied heat from collectors located through the housing project. The major advantage of a centralized system is the reduced unit costs due to the scale of the project, when all houses have collectors. A further advantage is the small tank heat loss because its large volume results in a very beneficial surface-to-volume ratio. This benefit is offset, however, by the serious heat loss from all the pipe runs. This is especially critical in summer when the losses are the same but the demand for heat is small. It is also serious in winter, given the small space heating demand of very low energy housing.

References:

Faninger, G. (2004s) Solar Thermal Systems with Water Collector, available at www.energytech.at

Faninger, G. (2004b) Market Deployment of Solar Housing in Austria, available at www.energytech.at

Faninger, G. (2004c) Solar Supported District Heating for Housing Estates, available at www.energytech.at

Weiss, W. (2003) Solar Heating Systems for Houses: A Design Handbook for Solar Combi-systems, James and James Ltd, London

Solar system planning tools

Meteonorm: METEOTEST, Fabrikstrasse 14, CH 3012 Bern, Switzerland, [email protected], http://www.meteotest.ch

Polysun: Institut für Solartechnik SPF Hochschule für Technik Rapperswil (HSR), Oberseestrasse 10, CH 8640 Rapperswil, Switzerland, [email protected], www.solarenergy.ch

SHWwin: Zur Auslegung von Brauchwasseranlagen, Teilsolare Raumheizung, Nahwärmenetze, Institut für Wärmetechnik, TU Graz (only German version available), www.wt.tu-graz.ac.at

TSOL: Dr. Valentin Energie Software GmbH, Stralauer Platz 33-34, D 10243 Berlin, Germany, [email protected], www.valentin.de

Websites

International Energy Agency (IEA): www.iea.org

International Energy Agency (IEA) Solar Heating and Cooling Programme (SHC): www.iea-shc.org IEA SHC Task 26 homepages: www.solenergi.dk/task26/downloads.html

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