Solar Energy

SOLAR ENERGY A FEW FACTS Every day the earth receives thousands of times more energy from the sun than is consumed in all other resources. Eg. if a 140x140 mile parcel of land in Arizona was covered with solar cells, the electricity needs of the entire United States could be met. The sunlight falling on a typical house can provide from 1/3 to 1/2 of the heating needs of that house. Today solar energy accounts for only 1% of the total renewable energy consumed in the United States

Characteristics of Isolation Isolation is the amount of solar radiation reaching the earth. Also called Incident Solar Radiation. The sun’s energy is created from the fusion of hydrogen nuclei into helium nuclei. Components of Solar Radiation:  Direct radiation  Diffuse radiation  Reflect radiation

SOLAR HEATING TODAY Used mostly for heating pools and domestic hot water (DHW) Two types of solar heating systems:  Active Solar Heating System  Passive Solar Heating System

ACTIVE SOLAR HEATING SYSTEM A system that uses water or air that the sun has heated and is then circulated by a fan or pump. Three Types:  Flat Plate Collectors  Batch Water Heaters  Thermosiphon

FLAT PLATE COLLECTORS A thin flat metal plate is used to absorb the sun’s radiation. Tubes carry water into the absorber plate where it is heated by the sun and sent to a pump or fan into storage and distributed from there to the living space.

BATCH WATER HEATERS Pre-heats water using the sun by having a black tank inside an isolated box with a glass cover. Solar energy is absorbed within the box to heat the water. The water outflow is sent into a conventional water heater for further heating. They are also called “Bread- Box” heaters.

THERMOSIPHON This method places the storage tank above the solar collector. Cold water is put into the bottom of the storage tank where it is circulated through a flat plate collector and pumped back into the top of the storage tank. The heated water can then be taken from the top and used.

PASSIVE SOLAR HEATING SYSTEMS The house itself acts as the solar collector and storage facility. No pumps or fans are used. This system makes use of the materials of the house to store and absorb heat. Three Types:  Direct-Gain  Indirect-Gain  Attached Greenhouse

DIRECT-GAIN Large south facing windows that let in the sunlight. Thermal mass is used to absorb the radiation. At night the absorbed heat is radiated back into the living space.

INDIRECT-GAIN Collects and stores the solar energy in one part of the house and use natural heat transfer to distribute heat to the rest of the house. Popular method is to use a Trombe Wall which is a massive black masonry that acts as a solar collector and a heat storage medium.

ATTACHED GREENHOUSE Uses a combination of Direct and Indirect-Gain systems that use water drums and a masonry floor as heat storage in the attached greenhouse. Thermosiphoning can use direct-gain from the flow of air created by the difference in pressure between the less dense warmer air of the room and the cooler air near the ground.

THERMAL ENERGY STORAGE Solar energy heating systems must be able to store energy for night time use and for cloudy days. Different materials absorb different amounts of heat. Depending on the weather and the amount of thermal energy stored will determine how long a house can continue to be heated by the stored solar energy. Phase-change material can be used to add additional heat to the living space.

This House Uses an Active Air System

Typical Indirect Solar Domestic Hot Water System

Positioning of Collectors Rule of Thumb For a year-round solar system in Canada, the best performance is from collectors facing due south, tilted at an angle equal to the latitude of the location and never shaded. For a summer-only system, such as pool-heating, the tilt angle will be at a shallow angle. If most of the use is in the winter, the panels will be at a steeper angle to capture the sunlight at lower sun altitude angles. Summer Biased: Latitude – 15o Winter Biased: Latitude + 15o

Solar Collector Designs The task of a solar collector is to achieve the highest possible thermal yield. Different collector designs

Solar Collector Designs Concentration of solar radiation Concentration of solar radiation: single reflector with two-axis tracking Concentration of solar radiation: multiple reflectors with two-axis tracking

Geometry of Solar Collectors The gross surface area (collector area) is the product of the outside dimensions, and defines for example the minimum amount of roof area that is required for mounting. The aperture area corresponds to the light entry area of the collector – that is the area through with the solar radiation passes to the collector itself. The absorber area (also called the effective collector area) corresponds to the area of the actual absorber panel. Cross-section of a flat-plate collector

Reference Area When comparing collectors, the reference area is important – that is, the surface area from which the collector’s characteristics values are drawn. For North American ratings, the reference area is equal to the gross area. For the energy yield, it is not the collector (gross) area that is crucial but the absorber area. The exception to this is evacuated tube collectors with reflectors.

Geometry of Solar Collectors Cross-section of a heat-pipe evacuated tube collector with description of the different surface areas

Unglazed Collectors Unglazed collectors no glazing or insulated collector box consists only of an absorber (usually made of plastic) high thermal losses, used only at low operating temperatures typically used for swimming pools inexpensive

Unglazed Collectors

Glazed Flat-Plate Collectors

The absorber consists of a heat conducting metal sheet (made of copper or aluminum, as a single surface or in strips) with a dark coating. The tubes for the heat transfer medium, which are usually made of copper, are connected conductively to the absorber.

Glazed Flat-Plate Collectors Most spectral-selective layers have an absorption rate of 90-95%, and an emission rate of 5-15%. Commonly used selective coatings consist of black chrome or black nickel.

Glazed Flat-Plate Collector Advantages offers multiple mounting options good price/performance ratio cheaper than vacuum collector Disadvantages lower efficiency than vacuum collectors for high temperature applications, because the heat loss coefficient is higher not suitable for generating high temperatures (+100oC) requires more roof space than vacuum collectors do support system is necessary for flat roof mounting

Vacuum Collectors Evacuated tube collectors In order to suppress thermal losses through convection, the volume enclosed in the glass tubes must be evacuated to less than 10-2 bar. Additional evacuation prevents losses through thermal conduction. The adiation losses cannot be reduced by creating a vacuum. The losses are kept low by selective coatings (small ε value).

Vacuum Collectors Advantages achieves a high efficiency even with large ΔT’s between absorber and surroundings supports space heating applications more effectively than do glazed flat-plate collectors low in weight, can be assembled at installation site Disadvantages more expensive than a glazed flat-plate collector cannot be used for in-roof installation heat pipe systems need to be inclined at least 25O

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