Design a solar hot water system for Los Angeles to satisfy the requirements from

Question

Design a solar hot water system for Los Angeles to satisfy the requirements from the Solar Decathon 2017 Rules (below). Describe the design, in terms of solar collector size, heat exchanger assumptions and water storage. Neglect any losses in pipes between the water heater and the water tap.

Hot Water For each draw, at least 15 gal (56.781 L) of hot water shall be delivered in no more than 10 minutes to be eligible for points. All available points are earned by delivering an average temperature of at least 110°F (43.333°C)

Explanation / Answer

In a hot-water heating system for Los Angeles, water is used to store heat energy and to carry it from the burning fuel to the location where the heat will be used. All hot-water (hydronic) systems consist of these basic parts:
i. A water tank, in which the heat is absorbed and stored;
ii. A pump and piping system to transport the heated water;
iii. A heat exchanger to release the heat where it is needed;
iv. A control system to control the rate at which the heat is used

Collector size:

In a typical solar water heater, water is heated by the solar thermal energy
absorbed by the collectors. The hot water with lower density moves upwards
and cold water with higher density moves down from the tank due to gravity
head. A bank of collectors can be arranged in a series – parallel combination
to get higher quantity of hot water. A typical litres insulated tank with a
2 square metre collector area, will supply water at a temperature of 100 - 120°F.

Heat Exchanger :
An important part of any hot-water system is the heat exchanger or radiator.
If it is not sized correctly or the airflow through it is insufficient, the
performance of the system can suffer greatly. Fortunately, heat exchangers
come in many different sizes. A wide range of commercial radiators designed
specifically for hot-water systems are available. Most of these can operate
at water pressures as high as 50 to 60 psi and have threaded fittings for
connecting them to the distribution system. A very suitable alternative to a
commercial radiator is a new or used automobile radiator. These are available
in many different sizes and can be purchased at most junk yards and parts supply
houses. Many dealers have new radiators for older cars that they might be willing
to sell at reduced prices. Automobile radiators, however, are not generally
suitable for water pressures higher than 15 to 20 psi. This limitation should not
be a problem if the pump and distribution pipes are sized correctly. Automobile radiators,
however, will need some modifications, including plugging of the fill and overflow
holes and modifying the transition from the rubber hose fitting to the distribution pipe.
The heat transfer characteristics of any radiator are dependent on a number of factors.
The most important are the flow rate and temperature of the water and air streams.
In general, the greater the temperature difference between the water and the air,
the more rapidly heat is transferred. Also, the more water and air passing through
a radiator, the greater the amount of heat transferred. Factors such as the design
of the radiator, the number and distribution of fins, and the material from which
the radiator is made are also important. For example, under typical operating conditions
many commercial heat exchangers designed specifically for hot-water service deliver
about 20,000 BTU per hour for each square foot of face area. Since most radiators have similar heat transfer characteristics, the deciding factor in determining capacity is physical size. Tests have shown that automobile radiators can transfer between 16,000 and 20,000 BTU per hour per square foot of face area.
For example, a radiator that is 112 feet wide by 2 feet tall has 3 square feet of area.
It could therefore transfer between 48,000 and 60,000 BTU per hour.

Water storage :
The most visible part of a hot-water system is the water tank. Standard tanks suitable
for hot-water heating systems are available in a variety of sizes, capacities, and wall
thicknesses. Underground tanks have thicker walls than the above-ground variety,
making them much better for welding. Given a choice, it is better to use a short,
large-diameter tank than a long, thin one because a shorter tank has less surface area,
reducing heat loss and cost of insulation.The tanks are generally made of stainless steel to avoid corrosion and are
insulated to reduce heat losses. They are also fitted with electrical heater as
a backup during monsoon days. The tanks may also be made of G.I.

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