Title: Hot Water and Solar Hot Water
1Hot Water and Solar Hot Water
- Dr. William J. Makofske
- Sustainable Warwick
- Warwick Town Hall
- October 29, 2008
2Household 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..
3The 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.
4 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.
5Other 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
6Batch 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.
7Thermosyphoning Systems in Greece
- The tank sits above the collectors. Hoses
bring water to and from the tank. This is a
non-freezing climate.
8Thermosyphoning 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.
9Active 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.
10Solar Collector
11Solar Collector Pipe Shape
- Typical shapes for the collector pipes inside the
box are a parallel configuration (top) or a
serpentine configuration (bottom)
12Types 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.
13Drainback Collector Systems
- To prevent freezing, the collector water drains
automatically when the pump shuts off. This is
more reliable than the draindown approach.
14Closed 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.
15Single 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.
16Evacuated Tube Collectors
- Vacuum tubes reduce heat loss from the
collector. They are generally more expensive and
have shorter lifetimes than other collector types.
17PV- 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.
18PV-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.
19System Diagram
- PV Assisted Solar Hot Water
- Heat exchanger transfers heat from antifreeze
solution to solar storage tank by thermosyphoning
20Optimal 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)
21Why?
- 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.
22But 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.
23Economics 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
24Typical 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
25Solar 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.
26Parabolic 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.
27Parabolic 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.
28Credits
- 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
29The 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
30Convection 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.
31Thermosyphoning Systems I
- Uses a solar collector to circulate hot water to
a storage tank - No pumps needed hot water rises naturally,
cooler water falls
32Thermosyphoning 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.
33Active 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
34Draindown 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.
35Other Collector Systems
36Typical 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
37Performance 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)
38Sizing 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.
39Size 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.