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Hot Water and Solar 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. ... – PowerPoint PPT presentation

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Title: Hot Water and Solar Hot Water

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
  • 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
  • Solar hot water is a viable option..

The First StepConservation Efficiency
  • 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
  • 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.

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

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
  • 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

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)

  • The suns 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 (lt
    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
  • 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
  • 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 suns
    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 suns 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.

  • PV driven solar hot water pictures by W. Makofske
  • Solar passive water heater in Greece taken by W.
  • Other pictures from NREL,National Renewable
    Energy Laboratory

The Batch or Bread Box System
  • Advantages simple, cheap, home-built, no pumps
  • 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

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
  • 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
  • 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.
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