Hot Water and Solar Hot Water Dr. William J. Makofske Sustainable Warwick Warwick Town Hall October 29, 2008 Household Hot Water • Hot Water consumption – 20 gal per person per day • Daily basis for showers and baths, for washing dishes and clothes, as well as other purposes. • For a family of 4, it will consume around 200 gallons of oil a year (assuming a 70% efficiency). • About 25% of home energy consumption • 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 is a viable option.. The First Step–Conservation & Efficiency • Reduce Demand: • Since solar hot water systems • Low-flow shower heads are not cheap, it makes • Add faucet aerators economic sense to reduce hot • Lower water-use clothes water use and improve washers and dish washers efficiency of use so that the • Take shorter showers solar system can be the smallest possible size to meet your • Reduce Heat Losses needs. • Insulate hot water tank • Conservation and efficiency are • Insulate hot water pipes usually the cheapest approaches • Efficient Technology to reducing energy use. • Efficient water heaters • On-demand water heaters • Solar water heaters Solar Hot Water • Will supply about 75% of your hot water needs over the year if sized properly. • . • Almost all solar hot water heaters use an auxiliary backup system when the sun is insufficient. 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 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. 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 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. Active Solar Hot Water Systems Collectors Solar water storage system Pump(s) Heat exchangers Controls DIRECT where water is pumped directly through the collector and back into the storage tank, INDIRECT where an anti-freeze fluid is pumped through the collector, and heats water in storage by means of a heat exchange coil. Solar Collector 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 • 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. 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 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. Single Tank System • 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. Evacuated Tube Collectors Vacuum tubes reduce heat loss from the collector. They are generally more expensive and have shorter lifetimes than other collector types. PV- Driven Solar Hot Water • Two 4 x 8 ft collectors and a small 15 watt PV unit • 80 gallon storage tank and a small heat exchange and DC pump unit. 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 pump flow is directly proportional to the solar energy available. System Diagram • PV Assisted Solar Hot Water • Heat exchanger transfers heat from antifreeze solution to solar storage tank by thermosyphoning Optimal Siting of the Collector • Optimal positioning for a solar hot water collector is • facing due south with • tilt angle equal to the latitude of the site.(40 degrees for Warwick) Why? The sun’s path is symmetric with respect to the south direction Collector tilt angle roughly midway between summer and winter so you get decent collection throughout the year. But Non-Optimal Siting OK • Not highly sensitive to the exact orientation and tilt of the collector. • 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. Ground mounting is ok, too. Economics of Solar Hot Water The economics of solar hot water will depend on • The price of the solar system (and subsidies) • The lifetime of the solar system (25 yrs) • Maintenance costs • The cost of heating the water with auxiliary energy • Projections of increasing costs of energy Typical Payback Economics • Assuming an out of pocket cost of $3000 for a system that supplies ¾ of the hot water demand of 80 gallons a day., oil at $3.00 gal, and water heater efficiency of 70%, we have • Natural gas PT about 7-8 yrs • Electric PT about 5-6 yrs 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 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 troughs are most used in dry desert regions that have plenty of direct sunshine. Costs have dropped dramatically with research and development efforts. 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 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 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 • Uses a solar collector to circulate hot water to a storage tank • No pumps needed – hot water rises naturally, cooler water falls 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. 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 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. Other Collector Systems 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 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 • 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 • 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.