PowerPoint Presentation - Passive Solar Design

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					Hot Water and Solar Hot

   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
• 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
    Other Applications of Similar
•   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
Thermosyphoning Systems in
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
Solar water storage system
Heat exchangers

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
           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
       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
• Heat exchanger
  transfers heat from
  antifreeze solution to
  solar storage tank by
  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 sun’s path is
symmetric with
respect to the south
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
•   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.
• PV driven solar hot water pictures by W.
• 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
• Warm water and warm
  air are less dense
  compared to cooler
  fluids and rise by a
  process called
  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
     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
  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
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-
• 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
        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
• 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.