Ventilate, heat and generate hot water

MEMORY

Introduction

The purpose of this presentation is to describe the technology of ventilation, heating and generation of domestic hot water using air collectors. The use of air collectors is particularly interesting, as they represent a simple and effective support for heating.

It is preferable to work with air as a heat transfer fluid because:

· The air does not freeze, does not bulge and does not corrode like water, which reduces the risk of breakdowns.

· The air heats up very quickly, obtaining thermal gains even with minimal irradiation. Enough to keep any home heated and ventilated, and store enough hot water in summer to meet your demand.

· The installation of air circuits in an existing home is simpler than that of water circuits.

Although marketed in the USA. since the late nineteenth century, air collectors are relatively little known compared to water collectors, accounting for only 1% of the total global solar thermal market. While the Romans were already designing and building water supply networks with lead pipes, the science of ventilation developed practically in the last 100 years. Any house has a drinking water network and a sewer network, some have rainwater evacuation and irrigation systems, but very few have active ventilation systems.

Technological background: The greenhouse effect

Although glass was invented in Egypt or Mesopotamia, it was applied only in jewelry and tableware. The Romans were the first to make windows with sheets of mica or alabaster and create the heliocaminus (solar oven), the first greenhouse for plants from exotic countries. Later they improved them with the development of glass laminating techniques, whose production continued during the Middle Ages, as attested by the stained glass windows of the cathedrals and the first greenhouses with rose bushes or orange trees of the monasteries.

The noted French-Swiss naturalist Horace de Saussure observed in the 1760s that, "it is a known fact, and this has been for a long time perhaps, that a dwelling, a float or any other place always heats up more if the sun's rays have to pierce a glass." To determine the effectiveness in trapping heat with glass covers, de Saussure built a rectangular drawer, insulated its interior, placed two smaller boxes inside and had it covered with glass. After exposing it to the sun, it measured an increase in temperature in the inner boxes of 109 ºC, that is, higher than the boiling point of the water.

He repeated the experiment high in the Alps, recording the same temperature increase inside the box, from which he deduced that solar radiation was the same everywhere, but that "the less dense atmosphere at heights is less able to retain it." De Saussure considered that one day the hot box would have practical applications since it is "relatively small, easy and cheap to build".

In fact, greenhouses became fashionable in the nineteenth century to recreate tropical climates and raise plants brought from expeditions around the world. But also as a heating element integrated into homes, until then wet and cold. It is also at this time when great scientists from other disciplines, such as the French mathematician Fourier, the English astronomer Herschel or the American physicist Langley laid the foundations of the study of climate with the explanation of the greenhouse effect at the atmospheric level.

The American Edward Morse was the first to patent a solar collector in 1881 (US Patent 246.626), and used air as a heat transfer fluid. Through a glass it heated a wavy sheet painted black, and allowed the air to circulate by convection behind it and into the interior of the house. This system also allowed it to suck air from outside, creating natural ventilation.

Solar heating

Since its inception at the beginning of the twentieth century, the solar thermal industry must face two fundamental problems: the storage of energy for times when solar irradiation is not available, and the efficiency in its capture. Although when talking about solar thermal energy, our country usually thinks of the generation of hot water, the greatest thermal demand occurs in heating. Considering that any construction acts as a heat store, we continue with the problem of storing that energy at times when solar radiation is lacking (at night and on cloudy days).

For example, if a single-family home has a heating energy demand on the coldest days of 10 kW, it needs 240kWh/d on those days. With an average winter irradiation of 2.2 kWh/m² per day (depending on the area) a collecting area above 100m² would be needed to meet the demand. To store the energy needed only to get past the night, you would need a tank of 160 kWh for example: 4000 lts. of water at 60°C (ΔT=35K).

Although technically it would be possible to solve it, we immediately realize that covering the demand for 100% solar heating is not feasible or reasonable.

That is why the use of solar systems as a support in heating has been established, reducing the period of use of conventional systems to the coldest days. Although the solar fraction seems reduced in heating systems (up to 30%) compared to DHW systems (70%), the absolute energy saved is very high and therefore interesting for users.

Taking into account that solar air systems ventilate the house with overpressure, significantly reduce losses due to infiltration (lack of tightness), this effect is multiplied and not only counts the solar energy provided, but also the energy not lost.

Storing heat in tanks only allows the storage of a small part of the total demand. While the water systems heat the house only to the minimum comfort temperature, those of AireSolar do so to the maximum point, using the structure of the building itself as an energy warehouse. The difference between these temperatures multiplied by the caloric capacity of the building gives the heat storage capacity in the building.

To obtain good results in the solar system you have to find a way to have a high value. And this can be done by improving the storage capacity of the building, or by improving the efficiency of the solar system. For this, technical solutions such as vacuum tube collectors have been pursued, or the direct heating of the air in collectors specially designed for this purpose.

AireSolar technology

Combining architectural criteria with the knowledge acquired about the greenhouse effect and the availability of effective high-performance ventilation equipment, direct solar air heaters, without water circuit, resurfaced during times of oil crisis of the twentieth century, specifically in the two important ones that existed:

· The shortage of supply suffered during the Second World War led to new developments such as the Trombe Wall, improvements in the design of heat flows and exchanges, and even solutions to the storage problem such as conducting hot air through gravel accumulators. It can be considered as the first installation of a complete AireSolar system that carried out by Professor George Løf of Colorado State University in his own home in 1945. Even today, after more than 50 years, it continues to contribute to the heating of the house.

Also noteworthy is Tucson, Arizona architect Arthur Brown, who made the first installation of AireSolar in a public building, the Rose Elementary School in his city. He created a glazed roof and connected fans to the space between the roof and the glass, thus managing to cover 80% of the school's heating needs.

· The second leap for AireSolar technology occurred in Europe after the oil crisis of the 1970s, when pioneers in Germany, Holland and Denmark demonstrated the viability of their facilities and developed integration into buildings combined with bioclimatic considerations. Today, completely self-sufficient homes in heating and DHW are already being built in Europe thanks to thermal air and water systems.

Some companies managed to develop marketable products from these experiences. In the US and Canada, perforated sheet metal panels are installed as a second skin in industrial buildings to take advantage of the heated air between the two layers of the façade (Solarwall). In Germany, Austria and Denmark, air collectors themselves are manufactured in an industrial way, modularly expandable for all types of applications, and even autonomous, with their own photovoltaic cell to power the fan (Grammer Solar TwinSolar series).

Air as a heat transfer fluid

Although in colloquial language we refer with the term fluid to liquids, in the field of physics any gas or liquid that adapts its shape to that of the container that contains it is defined as a fluid. So although air is a gas and water a liquid, both are fluids that share the same laws of fluid mechanics.

The fundamental difference between gases and liquids lies in their compressibility, that is, the ability of gases to vary their density in inverse relation to the pressure to which they are subjected. However, the operating speeds worked on in AireSolar systems can be considered a constant air density, so that the differences in the dynamic behavior between both fluids (air and water) are reduced.

One of the most important phenomena in the study of fluid mechanics is the transition from laminar to turbulent flow. The dynamic and thermal properties of the fluid change drastically so its knowledge is crucial. Although laminar flows are disadvantageous for heat exchange, the pressure loss implied by a turbulent flow is so important that it is avoided in all cases, limiting the circulation speeds of fluids in all systems (as with water).

Assuming, then, that the density of the air is constant and that the flow is laminar, the pressure losses in ducts (rectilinear and constant section) depend only on the speed of circulation, and on the quadratic relationship described by Bernouilli, applicable to both air and water:

Δp = 1/2 r v2

In the case of Grammer Solar air collectors, the accompanying diagram shows the relationship between the internal pressure loss and the air flow through it.

The dotted line indicates the usual operating flow rate in AireSolar systems and the consequent loss of load in the collector.

To these are added losses of 11.5 Pa at the inlet and outlet of the collector, as will be seen below.

In the system pipes, friction losses are much lower in the case of air than in the case of water, since the dynamic viscosity coefficient μ is about 50 times lower. That's why circulation speeds can be much higher.

Friction losses are divided into losses in the duct and losses in singularities (elbows, shunts, transformations, etc.) The calculations associated with both losses are very complex so tables supplied by the manufacturers of ventilation accessories are usually used.  Suppliers usually give sizing recommendations or sell packages with fan and integrated connections directly, which usually involve a compromise between necessary space and speed in the tube. Eventually you have to distribute the airflow directly after the fan to reduce noise.

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We can conclude that the qualities of air require special constructions with larger diameters. That can give problems in the concrete realization looking for a place for the components and the pipe. Taking them into account, these differences do not affect the efficiency of the solar heating system but can also increase performance.

Although there are already computer programs that simulate pressure losses in complex air distribution networks, experience shows that in most domestic applications simple estimation rules can be applied:

· Maintain moderate speeds between 3 – 5 m/s (the maximum allowed to control noise in residences).

· Minimize and, if any, attenuate any speed variations.

· Avoid blunt edges in any direction of circulation.

· Maximize radii of curvature in changes of direction.

Air as a heat carrier medium

As in other fluids, the power of the Pc collector corresponds to the power of the energy transport Pe:

Pc = Pen0*G –a1*ΔT – a2*ΔT2 = f*cv*ΔT

where h0, a1, a2 are specific values of each collector, G is the solar irradiation, f is the flow of fluid through collector and cv is the specific heat of the heat carrier.

But there are two basic differences in the thermal behavior of air that have important consequences:

· the low volumetric specific heat of the air causes it to heat up faster than water, reaching the desired operating temperature even with low irradiation (covered sky).

If we take one m3 of air and 1 m3 of water and each one we provide the energy of 1 kW, while the water will be heated 1 ºK, the air will be heated 3,488 ºK. This means that with little energy input we obtain a much greater thermal jump, and that to transport it we have to work with higher flows, increasing the diameter of the duct or the speed of circulation. In the case of Grammer Solar collectors, the usual ducts and operating speeds are shown in the following table:

The low caloric conductivity of the air influences the construction of the collector. On the one hand, resting air represents the best insulator (after vacuum), but when it is in motion it improves its conduction. Hence, when it is windy we feel cold, and we bundle up to keep a layer of air still on our skin. All this influences the way in which the collector is built, and the arrangement of the absorbers with respect to the circulating fluid. There are several alternatives on the market:

In this presentation, the selected product is the Grammer SLK collectors, of type F, which reaches a very high efficiency. It takes advantage of the insulating capacity of the air by maintaining an air chamber at rest between the absorbers and the glass, while the air flows regularly and rectilinearly through the absorption channels.

They have a very large range of optimal efficiency so they can also be used with modulated fans, obtaining the optimal temperature for the system application:

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We can conclude that the best arguments in favor of the implementation of AireSolar systems refer to their operational behavior:· AireSolar systems have fewer corrosion problems, so the operational life of collectors and other components is longer than in systems that use water.· AireSolar systems do not need antifreeze or anti-boiling protection, safety valves or expansion vessels.· The warm air used in the circuit can be driven directly into the house without going through any energy exchange, thus reducing reaction time such as losses; and allowing to work with lower temperatures and thus improve yields.· AireSolar systems do not require pumping groups or expansion vessels, elements prone to breakdowns and requiring professional maintenance.· Distribution ducts do not require absolute tightness. Small leaks have little effect on system performance and also do not affect construction.· It attaches directly to the air of the house, which is always the goal of all heating.

· Heating the air directly should not touch the traditional system (electric stoves, radiators, fireplaces, etc.)

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INSTALLATION PROJECT

Building description

The house object of this project is located in the interior of the province of Valencia, in the basin of the Turia River, an area that currently belongs to the municipality of LLíria.

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Radiation tables

The Luftikuss program, supplied by Grammer Solar, uses the radiation tables of PV-SOL, considered one of the most reliable sources at European level. If we compare the data with the radiation tables published by the Valencian Energy Agency AVEN, we observe a slight error throughout the year, although the total annual radiation is very similar.

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Sizing of the collectorsManual calculationsUsing the grammer Solar sizing graph, we see that to heat a house of about 110 m2 we will need a collecting area of between 8 and 10 m2. Considering that the demand for heating is not in all rooms, that it is not extreme and that both the budget and the available surface on the roof are limited, we will try to cover the maximum possible of the real energy demand with the minimum possible surface. Let's check now that with this surface the energy demand for the generation of DHW can be covered. The demand for ACS at 45 ºC for a single-family house of three members in Valencia is 157 l/day. If the water inlet temperature of the network is considered to be 10 ºC, we can calculate the daily energy demand as:

E = m • Cp • ΔT

dondem : amount of water in kg. Cp : specific mass heat of water = 1.16 Wh/kgKΔT : thermal difference in ºK between cold and hot water

E = 157 kg • 1.16 Wh/kgK • (45-10) = 6374 Wh/day

If we consider that the months of use of the air collectors to generate DHW will be mainly from May to September (Summer in the Northern Hemisphere), we will take the average value of the radiation during those months (6217 Wh / m2) to calculate the collecting surface necessary to cover the demand for DHW with a safety factor of 1.5 (since in Valencia we have few cloudy days): AK = 1.5 • E /( Is • n )

dondeIs : average daily solar radiation on the collecting surface, in kWh/m²n : average solar system performance (for DHW a value of h = 0.47 applies)

AK = 1.5 • 6374 Wh / (6217 Wh/m² • 0.47 ) = 3.27 m²

Then with 8.5 m2 of collecting area left over to cover the entire demand for DHW in summer, (with the advantage that we can not suffer overheating as in a water collector).

We are within the internal parameters to heat the house in winter. Although the minimum would be a TopSolar 8.0, due to its elongated arrangement it would not fit on the roof of the house and we propose this U-shaped version.

Calculations with software

To size the system we can also use the Luftikuss software supplied by Grammer Solar, an application similar to those used in water-based systems, which is included in the first installation and can be requested from our representative in Chile.

Grammer Solar generally recommends a working flow rate of 350 m3/h in AireSolar systems. As we will see later in the description of its technical characteristics, in the case of a TopSolar 8.5 system with water circuit, a very powerful fan is recommended (beats 250 Pa with 350 m3 / h) to ensure the correct transfer of air in both operating modes.

The pressure losses caused by the collectors themselves and their inputs and outputs can be obtained from the following curves:

With 350 m3/h we suffer load losses of 9 Pa/m of collector and 12 Pa per input/output, adding a total of 9 * 8.5 + 12 * 2 = 100.5 Pa. To this we must add the losses of the circuit in each mode of operation:

·126 Pa in ventilation mode: Filter box (15), anti-return clapper (1), 4 tes (60), 25 meters of duct (25), silencer (10), motorized valve (15).

·98 Pa in ACS mode: 1 te (15), 8 meters of duct (8), silencer (10), motorized valve (15), air/water exchanger (50).

DESCRIPTION OF MAIN COMPONENTS

Collectors

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The collector presented is the SLK model, the third version launched by the company Grammer Solar in 2003 after passing the relevant tests at the Fraunhofer Institut für Solare Energiesysteme (Freiburg, Germany) and the Arsenal Research (Austria), two of the most recognized organizations worldwide in solar research.

The performance curve was measured according to the procedures defined in the draft regulation prEN 12975-2 and at the Fraunhofer Institute. Annex 1 includes the Technical Data Sheets of the series.

The characteristic values of the collector are:

The housing is made of aluminum and the basic dimensions of the collector are 2000x1006x136 mm. The weight of each collector is 45 kg., so the set used in this project (TopSolar 8.5) has a weight of 205 kg. The air-conducting absorbers are also made of 0.6 mm aluminium painted with black chrome. Its total capacity is a volume of 55 liters of air. The collector cover is made of 4 mm ESG glass.

The thermal insulation is made with mineral wool of 50 mm at the rear and 20 mm at the sides, with a thermal conductivity of l = 0.04W / (mxK).

Fan

The fan incorporated in the model is the low-pressure radial fan, model ENG 3-9.8 from the manufacturer Karl Klein Ventilatorenbau GmbH. Its characteristic values are:

· Working voltage 230 V a.c.· Rated current 0,90 A· Maximum flow rate 500 m3/h· Maximum pressure jump 450 Pa· Nominal revolutions 2,760 min-1 · Power 120 W· Noise 67 dB (A)· Weight 4.5 kg.

Below are its characteristic curve:

Filter Box

The filter box is a square box of 265x265x235 mm of galvanized sheet on both sides, which is coupled to the 160 mm air pipes by means of the rubber joints at each end.

The box has a side slot to make the filter change.

It causes pressure losses of 15 Pa.

Silencer

The silencer is an acoustic attenuator from the German manufacturer Lindab, with inner and outer diameters 160 and 260 mm, 900 mm in length and 8 kg in weight.

It causes 10 Pa of pressure loss.

In projects with less available space, flexible silencers can be placed.

Air-water heat exchanger

Composed of aluminum sheets and copper tubes in a galvanized steel container, it is like a car radiator, facilitating an optimal exchange of heat between air and water.

Copper pipe connections are 3/4". It is designed for air flows between 300-500 m³/h, and causes 50 Pa of pressure loss.

As stated above, the complete ACS system operates with an efficiency of 47%.

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Regulation system

The DeltaSol Pro of the RESOL brand is an electronic regulator of temperature differentials very common in solar systems. It allows the regulation of two storage systems: one the house itself, and the other the water accumulator.

It stands out for its ease of use, thanks to a screen with 4 lines and a rotary button. Both facilitate access to very simple menus and the selection of options in an ergonomic and visually pleasing way.

As in any thermostat, the user selects the desired temperature TD. If Tv

If there is still radiation and the house is already at the desired temperature, DeltaSol Pro compares Tc with TA. And if Tc> TA, the R2t relay repositions the motorized valve and turns on the pressure group. Thus, the heat generated passes to the DHW system through the air-water exchanger.

INSTALLATION DESCRIPTION

PlansHouse scheme

There are no exact plans of the house, so the real surfaces of each space have been measured. The following diagram shows the distribution of the rooms as well as the dimensions of the proposed air distribution circuit.

The system is configured in such a way that two forms of operation are allowed:

· With the motorized valve in the heating position, the air is sucked by the fan through the filter box and the manifolds to be driven through the silencer into the house.

· With the motorized valve in the DHW position, the air is driven through the air-water exchanger, thus closing the retention key and returning the air to the collectors after having transferred part of its heat to the water.

Assembly

The assembly process has been as follows:

i. The roof is drilled in the positions indicated in the installation plan to place the insulation that protects the passage of the suction tubes and air impulsion through the roof. Being a system with air recirculation, 2 grommets are needed, one for suction and one for impulsion

iii. Then the clamping plates are fixed to the bolts, and the profiles are coupled to the plates, without fixing them yet until we mount the collectors on them.

The grommets are also placed to ensure the impermeability of the installation. Around each insulated perforation the tiles are raised a little to wedge the lead plate well between them and a section of tube is placed through the grommet.

iv. The collectors can now be hoisted to the roof. 0

Now the collectors are placed on the transverse guides and the sliding fixators are placed in position to coincide with the corresponding guide. In addition, the nozzles of the collectors must match the grommets.

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The fastening nuts are fixed between the profiles and the plates with electric or manual pressers, and the collectors are joined together by means of quick closures.

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Here we proceed to the assembly of the various elements of the technical circuit. First the filter box, the retention clapper, the shunt T and the air-water exchanger are connected:

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The air installation is finished by connecting the suction and impulsion to the collectors by means of the insulated flexible tube:

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In the basement we find a usual DHW installation, with its pressure group, expansion vessel, accumulator and copper valves.

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v. Finally the distribution of the air ducts above the support structure of the false ceiling is completed.

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Once the entire installation is finished, the plasterboard plates and diffusers can be placed.

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Final results

The AireSolar system presented (TopSolar 8.5 with DHW) has two operating modes with the aim of producing usable energy for as long as possible throughout the year. In times when heating is required, it produces it directly by heating the outside air and propelling it into the interior of the house. When the heating demand is met, the system switches to DHW mode to accumulate the thermal energy produced. (This makes sense since in heating mode it works at lower temperatures, and therefore with higher performance).

Thus, on a winter day the outside temperature can be 10 ºC while the one programmed with the regulator for the interior of the house is 20 ºC. The motorized valve automatically moves to the ventilation-heating position. In this way, the air passes through the collectors, the silencer and the distribution ducts inside the house until it reaches the desired temperature.

Depending on the cloudiness of the day, two things can happen:

· The house reaches 20 ºC thanks to the air collectors. Once reached, the motorized valve changes position to ACS mode and the fan recirculates the air through the collectors and the air-water exchanger, heating the primary water circuit that reaches the interaccumulator.

· The weather is adverse and the hot air system only reaches 17 ºC inside the house, with ventilation and reduction of relative humidity.

If users are not in the house, it remains heated and ventilated to reduce the thermal jump they have to overcome upon arrival.

When users arrive, they can add heat by lighting the fireplace (or any other additional system). Thus they notice an important energy saving, and greater comfort due to the oxygenation of the living room.

Maintenance

Air collectors

The Grammer Solar air collector has no moving parts, making it safe and maintenance-free, yet it is exposed to wind forces. To ensure that all threaded joints are in good condition, routine checks should be carried out, especially after storms.

Experience shows that collectors are normally cleaned by the action of rain; however, in heavily polluted areas or areas with a heavy pollen load, coupled with prolonged dry seasons, it may make sense to clean the glass cover with tap water and sponge or a broom to clean vehicles.

The collectors are protected against internal dirt with suction filters integrated into the system, according to category EU 4. The filter in the filter box must be changed once a year before the start of the heating season.

See original.