Solar Hot Water

1
Solar Hot Water

• Dr. William J. Makofske
• Ramapo College of NJ
• August 2004

2
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.

3
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.

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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.

5
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

6
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

7
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.

8
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.

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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

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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.

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Thermosyphoning Systems in Greece

• The tank sits above the collectors. Hoses
  bring water to and from the tank. This is a
  non-freezing climate.

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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.

13
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.

14
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.

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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

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Solar Collector
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Solar Collector Pipe Shape

• Typical shapes for the collector pipes inside the
  box are a parallel configuration (top) or a
  serpentine configuration (bottom)

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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.

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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.

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Drainback Collector Systems

• To prevent freezing, the collector water drains
  automatically when the pump shuts off. This is
  more reliable than the draindown approach.

21
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.

22
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.

23
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.

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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.

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System Diagram

• PV Assisted Solar Hot Water
• Heat exchanger transfers heat from antifreeze
  solution to solar storage tank by thermosyphoning

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Other Collector Systems
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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.

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Location of Collector

• The suns 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.

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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 (lt
  10) over the year. Collectors may also be
  mounted at an angle to the roof, although this is
  less aesthetically pleasing.

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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

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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

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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

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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.

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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.

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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.

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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)

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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.

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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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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