Solar varmtvannssystemer er designet for å overføre solens solenergi til vann. Å finne det mest effektive og effektive varmtvannssystemet for en gitt situasjon kan være en utfordrende oppgave. Det er en rekke nøkkelfaktorer som må vurderes når du velger den mest hensiktsmessige systemkonfigurasjonen. Disse faktorene inkluderer i stor grad mengden solinnstråling, klima, konstruksjon, installasjons- og materialkostnader, plassering og tilgjengelighet av systemet, mengde vann som trenger oppvarming, hyppighet av varmtvannsbruk, tilgjengelighet av elektrisitet, tilgjengelighet av materialer og ferdighetsnivå i konstruksjon.
Følgende klassifikasjoner av systemer er i tre grupper av to og en gruppe av ett unikt system. Disse fire hovedgruppene er:
- Åpen sløyfe vs Lukket sløyfe.
- Aktiv vs passiv.
- Bruker en varmeveksler vs Bruker ikke en varmeveksler.
- Batch system.
Et gitt system bruker en egenskap fra hver gruppe. For eksempel kan et system være et aktivt, åpent sløyfesystem som ikke bruker en varmeveksler. Eller et annet eksempel, et system kan være et passivt, lukket sløyfesystem som bruker en varmeveksler. Noen systemer er mye enklere å lage enn andre, og folk med grunnleggende kunnskap om verktøy og konstruksjon kan enkelt lage et funksjonelt system. Hvis man ønsker å lage sitt eget system, vil denne variasjonen i kompleksitet påvirke hvilken type system som velges.
Kostnad er en annen faktor, og hver systemkonfigurasjon kommer med en rekke forskjellige kostnader og fordeler. Kostnadene for et bestemt system kan variere mye fra land til land og region til region. Visse konfigurasjoner som bruker visse typer utstyr er mer effektive enn andre i spesifikke situasjoner. Følgende informasjon gir en grundig titt på disse ulike måtene å konstruere varmtvannssolfangere.
Ulike typer samlere er også vist på slutten av denne siden, samt eksempler på forskjellige vanlige solvarmesystemer.
Denne siden beskriver de ulike systemene som brukes til å varme opp vann med solen. For en mer generell beskrivelse av solenergi varmt vann besøk Solar varmt vann siden.
Innhold
Åpne sløyfesystemer
Open loop betyr at vannet som krever oppvarming strømmer direkte gjennom selve varmtvannssolfangeren. Det er ingen mellomliggende væske. Se Closed Loop-overskriften nedenfor for det ene unntaket fra denne definisjonen der vannet i et lukket sløyfesystem strømmer direkte gjennom solfangeren. Open Loop-systemer bruker ikke varmevekslere.
Fordeler
Open loop-systemer er de enkleste å designe og konstruere. De krever vanligvis minst mulig konstruksjon og har potensial til å vare lenger uten seriøst vedlikehold på grunn av deres begrensede mengde bevegelige deler.
Ulemper
Open Loop-systemer er ikke ideelle i områder der temperaturene kan falle under frysepunktet. Rørene kan fryse og få det flytende vannet til å utvide seg til is som vil sprekke rørene. Før frysende temperaturer må systemet dreneres for å bevare rørene. I områder der temperaturen faller under frysepunktet, anbefales det å bruke et lukket sløyfesystem med en varmeveksler eller implementere en eller flere av en rekke teknikker for å forhindre frysing av rørene. Forebyggende tiltak kan komme i form av systemavløp, enten manuell eller termostatstyrt, eller en pumpe som skyver konvensjonelt oppvarmet vann gjennom systemet i kaldt vær. Systemet kan også utformes slik at vannet dreneres ut av kollektoren ved tyngdekraften når sirkulasjonspumpen ikke går.
Closed loop systems
Closed loop systems are more complex to make and implement than Open loop systems as they can be used in a wider variety of climates. A closed loop system uses some kind of liquid that has a much lower freezing point than water, yet readily absorbs and releases heat. Antifreeze used in automotive radiators is a good example. This fluid passes through the solar collector, becomes heated, and moves into some type of heat exchanger. See Heat Exchanger Systems below. Due to the nature of Closed loop systems, Heat Exchangers are needed to transfer the heat from the collector to the water. Closed loop systems can also be used in radiant flooring or wall radiator applications to heat an area rather than water.
Advantages
Unlike most Open Loop systems, Closed Loop systems can be used in regions that experience temperatures below freezing. Due to the unique properties of water, pipes can burst when water freezes. This is prevented by using a fluid that has a much lower freezing point than water.
Disadvantages
Closed Loop systems require the use of a heat exchanger. This adds inefficiencies to the system that must be taken into consideration during the design and system sizing stage. Extra materials for the heat exchanger can also raise the cost of a system.
Active systems
Active systems are those that require external energy to function. Generally this energy comes in the form of an electric pump that pushes the fluid through the collector. It is not uncommon to find solar powered pumps as the external input. Solar pumping is appropriate in many circumstances due to the fact that the water is only being heated, and thus only needs to be pumped, when the sun is shining.
Advantages
Due to the nature of hydrology and thermodynamics, pumping of water through the collectors replaces the need to store the heated water above the collectors which can be challenging and restrictive to design (See Passive Systems for further explanation).
Disadvantages
The use of a pump requires both the purchasing of a pump, as well as the need for a power source, most likely electricity, to power the pump.
Passive systems
Also known as Thermosiphon System, Passive type solar systems are those which do not use an outside power source. Passive Solar is a term that is applied to much more than just solar hot water. For example, painting a swimming pool black so as to absorb more energy, or putting windows on the south side of a house (in the northern hemisphere) in order to let in more of the sun's energy are both considered Passive Solar.
A Passive Solar water heater must take into account the physics surrounding hydrology and thermodynamics. This type of system uses a thermosiphon. A thermosiphon uses the energy of the sun to move water or other collector fluid in a vertical direction. This occurs when a liquid has varying temperature, and thus density differences. Because the hottest water will always move upward in, thus displacing the colder water downward, the hot water storage tank or Heat Exchanger must be above the solar collector.
Advantages
No external energy is needed in order to move liquid through the system. Less moving parts equals less things to break.
Disadvantages
If a hot water tank or heat exchanger is used, it must be positioned above the solar collector in order for the thermosiphon to flow. This can cause problems as most solar systems are on roofs of houses. Many design options are now available that facilitate the storing of hot water above the system.
Heat exchanger systems
Heat Exchangers are used in regions that experience temperatures less than freezing. Heat Exchangers themselves are built into a closed loop system and transfer the heat gathered in the collector to the house's hot water supply. There are a number of designs for heat exchangers.
A common design is to supplement a conventional hot water heater by transferring the heat from the solar collector fluid into the hot water tank. Copper tubing is a good material for readily transferring heat energy of a liquid.
Another common style of heat exchanger uses a barrel with two lines of copper paralleling each other. One tube comes from the solar collector, enters the barrel at the top, spirals around the edge of the barrel, comes out of the barrel at the bottom, and completes the closed loop by returning to the solar collector. The other tube that parallels the first is the household hot water supply. It enters the barrel at the bottom, parallels the collector tubing to the top of the barrel, and exits the barrel at the top, with hot water. The heat is transfered from the hot fluid from the collector, through the copper tubing, and into the household water. The barrel can be insulated on the outside to increase efficiency of transfer.
There are a number of different fluids that can be used in the closed loop system. The following are the more widely used and readily available liquids: glycol/water mixtures, hydrocarbon oils, refrigerants/automotive antifreeze, and silicones. Each of these have benefits and disadvantages. Refer to this EERE reference link to find out more.[1]
Advantages
Heat exchangers are used for a couple main reasons: to protect the system against freezing, and as one option to supplement another source of water heating.
Disadvantages
Heat Exchangers add complexity to a hot water system. There are more aspects of a system that must be designed and planned for when using a Heat Exchanger. There is also a greater chance of things breaking or not functioning properly due to the added components. Caution must also be taken to insure that the collector fluid is either non toxic, or that there is very little chance of the collector fluid contaminating the potable water supply should there be ruptures in the closed loop system.
Non-heat exchanger systems
These are systems where the potable water to be heated flows directly through the solar collector. This type of system is an Open Loop system.
Advantages
See Open Loop System
Disadvantages
See Open Loop System
Batch systems
Also known as Breadbox, or Integral Collector-Storage (ICS) Systems, these include one or more black tanks or tubes in an insulated, glazed box. It could even be as simple as painting a tank black, putting it in a crate, insulating it on all sides except the one that is pointing at the sun, and covering that sun exposed side with glass or plastic. As the sun warms the water it expands and becomes less dense, encouraging the hot water to flow to the top of the collector. From there it is drawn out into the house as needed or into an auxiliary tank for storage. These work best in areas where freezing temperatures are rare. Despite their climate restrictions they work relatively well; in colder climates they can be used seasonally and drained before harsh winter months.
Collectors
Unglazed
Glazed
Evacuated tube
Batch collector
see http://commons.wikimedia.org/wiki/File:BatchSolarHeater.JPG
Examples
Active closed loop system
Active open loop system
Passive closed loop system
Passive open loop system
References
- ↑ US Dept of Energy. EERE Consumer Guide: Heat-Transfer Fluids for Solar Water Heating Systems. http://www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12940
http://www.builditsolar.com/Projects/WaterHeating/ISPWH/IPSWH.pdfDownload "The Integral Passive Solar Water Heater Book" by David Bainbridge