Solar Hot Water

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Information about Solar Hot Water

Published on November 9, 2007

Author: Octavio


Solar Hot Water:  Solar Hot Water Dr. William J. Makofske Ramapo College of NJ August 2004 Household Hot Water:  Household Hot Water Hot water is used in homes on a daily basis for showers and baths, for washing dishes and clothes, as well as other purposes. Average consumption per person is estimated to be around 40 gallons per person per day. For a family of 4, it will require Btu or the equivalent of gallons of oil to heat this water over a year (assuming a 70% efficiency). Over a year, it is the second largest use of energy in a home; only space heating is larger. Water is typically heated by a variety of fuels (oil, natural gas, propane) and also by electricity. All these methods use up valuable natural resources and create significant pollution. Solar Hot Water :  Solar Hot Water A method of supplying much of your hot water use is by producing hot water using the sun. There are a variety of ways to do that. Two ways to classify approaches are: Passive methods – batch and thermosyphoning designs using pump-free fluid circulation Active methods – methods that use pumps to circulate a fluid from collector to storage. However, this typically will not supply all your hot water since there are often several cloudy days in a row and storage is limited. Almost all solar hot water heaters use an auxiliary backup system when the sun is insufficient. The First Step–Conservation & Efficiency:  The First Step–Conservation & Efficiency Since solar hot water systems are not cheap, it makes economic sense to reduce hot water use and improve efficiency of use so that the solar system can be the smallest possible size to meet your needs. Conservation and efficiency are usually the cheapest approaches to reducing energy use. Reduce Demand: Low-flow shower heads Add faucet aerators Lower water-use clothes washers and dish washers Take shorter showers Reduce Heat Losses Insulate hot water tank Insulate hot water pipes Efficient Technology Efficient water heaters On-demand water heaters Solar water heaters Other Applications of Similar Technology:  Other Applications of Similar Technology Pool heating Space heating of buildings Absorption air conditioning Concentrating collectors for high temperature water for industry uses and for power production The Batch or Bread Box System:  The Batch or Bread Box System Advantages – simple, cheap, home-built, no pumps needed Disadvantages – less efficient than circulation models, freeze protection needed in winter, bulky, operator intervention often needed depending on weather conditions Batch Water Heaters:  Batch Water Heaters Batch water heater on a roof in Greece. Sun heats the tank in an enclosed insulated box with glazing. Greece has a non-freezing climate. Convection and Thermosyphoning:  Convection and Thermosyphoning Warm water and warm air are less dense compared to cooler fluids and rise by a process called convection. Thermosyphoning systems work on this principle. Thermosyphoning Systems I:  Thermosyphoning Systems I Uses a solar collector to circulate hot water to a storage tank No pumps needed – hot water rises naturally, cooler water falls Thermosyphoning Systems :  Thermosyphoning Systems However, the need to have the tank above the collector leads to some unusual hookup configurations. It also puts a tank of water that can leak at a higher position in the house. Thermosyphoning Systems in Greece:  Thermosyphoning Systems in Greece The tank sits above the collectors. Hoses bring water to and from the tank. This is a non-freezing climate. Thermosyphoning Systems :  Thermosyphoning Systems The main advantages are the lack of a pump and electrical energy savings. In warm climates, the tanks can be outside on the roof above the collectors. On slanted roofs, the tanks can lie horizontally on the roof itself. Evacuated Tube Collectors:  Evacuated Tube Collectors These collectors circulate water directly but do not freeze because of the low heat loss due to the evacuated space. They are generally more expensive than other collector types. Active Solar Hot Water Systems:  Active Solar Hot Water Systems The most common form of solar hot water system is the ACTIVE system, consisting of collectors, a solar water storage system, a pump, and controls to tell the pump when to go on and off. Such systems are often classified as DIRECT where water is pumped directly through the collector and back into the storage tank, and INDIRECT where an anti-freeze fluid is pumped through the collector, and heats water in storage by means of a heat exchange coil. Active Systems and Collectors:  Active Systems and Collectors There are many types of collectors but they mostly have the same features. Insulated box, glazed (glass or plastic) at the top to allow solar input Metal collector or absorber plate which has pipes for fluid flow connected to it Input and output connections Solar Collector:  Solar Collector Solar Collector Pipe Shape:  Solar Collector Pipe Shape Typical shapes for the collector pipes inside the box are a parallel configuration (top) or a serpentine configuration (bottom) Types of Active Systems:  Types of Active Systems Direct systems use only water in the collector. These are typically the draindown and the drainback systems. Indirect systems use anti-freeze circulated in the collectors. Some of these systems use standard pumps, and others use PV or solar-powered DC pumps to circulate the anti-freeze. These are typically called closed loop systems. Draindown Systems:  Draindown Systems To prevent freezing, a draindown collector isolates the storage system and drains the water in the collector when freezing temperatures threaten. Problems include loss of some water, and damage if the valves fail to operate properly. Drainback Collector Systems:  Drainback Collector Systems To prevent freezing, the collector water drains automatically when the pump shuts off. This is more reliable than the draindown approach. Closed Loop Systems:  Closed Loop Systems These systems typically have anti-freeze circulating in the collector loop with a heat exchange coil in the tank to prevent mixing of anti-freeze and water in case of leakage. This is the most common choice for a freezing climate. Closed Loop Systems:  Closed Loop Systems A single tank system typically uses electric elements for back up heating. The solar hot water rises to the top of the tank and the heating elements only go on if the temperature is below the thermostat setting. PV- Driven Solar Hot Water:  PV- Driven Solar Hot Water Two 4 x 8 ft collectors and a small 15 watt PV unit are the roof components. The basement components include an 80 gallon storage tank and a small heat exchange and DC pump unit. The heat exchange unit thermosyphons hot water into the solar tank while the pump circulates anti-freeze to the roof. PV-Driven DC Pump:  PV-Driven DC Pump The DC pump and motor sits on top of the heat exchanger and circulates an anti-freeze solution to the collectors on the roof. The anti-freeze drains down whenever the pump stops. The pump flow is directly proportional to the solar energy available. System Diagram:  System Diagram PV Assisted Solar Hot Water Heat exchanger transfers heat from antifreeze solution to solar storage tank by thermosyphoning Other Collector Systems:  Other Collector Systems Siting the Collector:  Siting the Collector Since you want hot water 365 days a year, the optimal positioning for a solar hot water collector is facing due south with the tilt angle equal to the latitude of the site. Location of Collector:  Location of Collector The sun’s path varies throughout the year. However, it is symmetric with respect to the south direction so that the most radiation is collected by facing south. By putting the collector tilt angle roughly midway between summer and winter, you get decent collection throughout the year. Non-Optimal Siting:  Non-Optimal Siting The amount of solar energy collected over the year is not highly sensitive to the exact orientation and tilt of the collector. For example, the collector could tilt between 30 and 50 degrees, or the orientation could be off from south by + or – 30 degrees with little loss (< 10%) over the year. Collectors may also be mounted at an angle to the roof, although this is less aesthetically pleasing. Economics of Solar Hot Water:  Economics of Solar Hot Water The economics of solar hot water will depend on The price of the solar system The lifetime of the solar system Maintenance costs The cost of heating the water with auxiliary energy Projections of increasing costs of energy Typical Payback Economics I:  Typical Payback Economics I Assuming a cost of $3000 for a system that supplies ¾ of the hot water demand of 80 gallons a day, oil at $1.80 gal, and water heater efficiency of 70%, we have Q = m c dT = (80 gal)(8.3 lb/gal)(1 Btu/lb0F)(70 0F)(365 days) Q = 17 x 106 Btu E = Q/eff = 24 x 106 Btu E(oil) = E/140,000 Btu/gal = 171.4 gallons Cost =$308.52 Savings = ¾ cost = $231.39 Payback Time = cost/savings/yr = $3000/$231.39/yr = 12.9 years Typical Payback Economics II:  Typical Payback Economics II However, many people use electricity to heat water. In the Northeast, at 15 cents per kw-hr, the economics for the same demand and solar system are: E = Q E(electricity) = 17 x106/3413 Btu/kw-hr E(electricity) = 4981 kw-hr Cost = 747.14 Savings = ¾ cost = $530.36 Payback time = $3000/$560.36 = 5.4 years Solar Concentrating Collectors :  Solar Concentrating Collectors Concentrating solar collectors focus the sun’s rays on a line (in a parabolic collector) or to a point (in a spherical collector). In both cases, the temperature of the receiver (the metal component enclosing a fluid) gets very hot. This is not needed for household use, but is desirable for certain industry needs and for producing electricity by running steam turbines. Parabolic Trough Collector:  Parabolic Trough Collector The parabolic trough collector has been used to produce solar electricity in many areas around the world. The tilt angle varies throughout the day to focus the sun’s rays on the pipe. Parabolic Collector Array:  Parabolic Collector Array Parabolic troughs are most used in dry desert regions that have plenty of direct sunshine. Costs have dropped dramatically with research and development efforts. Performance and Sizing- Collector:  Performance and Sizing- Collector A simple estimate of the size of the solar hot water system can be found from the following equation A(area in ft2) = solar fraction desired x Q(yearly demand in Btu) 200,000 Btu/ft2 From our previous example, assuming 75% of the load being provided from solar and a Q of 17 x 106 Btu Area = 0.75 x 17 x 106 Btu/200,000 Btu/ft2 = 64 ft2 Depending on the amount of sunlight available around the country, the solar collected per year could vary from 200,000 Btu/ft2 (NE) to 250,000 Btu/ft2 (SW) Sizing – Solar Storage:  Sizing – Solar Storage The solar hot water tank is typically 1-2 gallons of water for each square foot of collector area. A ratio of gallons of water to ft2 of collector often recommended is 1.5. For our system of 64 ft2 of collector, the storage tank would be about 64 ft2 x 1.5 gallons/ft2 or 96 gallons. Size the Collectors and Storage Tank:  Size the Collectors and Storage Tank A family uses 60 gallons of hot water per day. Assume the water is brought from 50 to 120 degrees F. Size the collector area and the storage tank size if the house is located in an area that provides 200,000 Btu/ft2 over the year. Credits:  Credits PV driven solar hot water pictures by W. Makofske Solar passive water heater in Greece taken by W. Makofske Other pictures from NREL,National Renewable Energy Laboratory

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