|
General
Information |
This guide is prepared
to provide basic technical information for large commercial or
industrial size solar water heating systems. All the points
mentioned in the guide have to be verified by M & E
engineers according to local requirements and would be their
responsibility. |
|
Preliminary
Load Estimate |
The
existing records of water heating fuel usage or hot water
consumption are an excellent source of thermal data and should
be used to estimate the load. If existing data or actual energy
use are not available an estimate should be made based on
typical use of hot water per day per person in the building.
When surveying a commercial or industrial site for possible
installation of an active solar heating system, it should also
be determined whether other sources of heat, such as gas
burners, waste heat recovery, are available at the site. |
| Physical
Constraints |
Once the construction of
the building has been confirmed, the best location for the
installation of the solar collector array as well as the storage
tanks should be determined. Weight load permitted at the
location where the collector array is to be installed must be
considered. This is particularly important for the storage
tanks. If possible, the storage tank should be located close to
the solar collector array. |
| Conceptual
System Selection |
An active solar water
heating system collects, stores and distributes solar energy
using liquid as the heat transfer media. The system includes
solar collectors, thermal energy storage tank, load interface
and system control unit and instrumentation. |
| Sizing
of the Solar Heating System |
a) Collector
area - Net absorber area of the collector could be
determined by making use of the local solar
radiation data.
b) Collector Slope and Orientation - The
preferred field orientation is facing true south (in the
Northern hemisphere), but deviation up to ± 30 º has minor
impact on solar energy system performance and is acceptable
to permit conformance with building orientation for roof-mounted
collectors and reduction in installation costs.
The preferred collector slope angle
is in the latitude.
c) Sizing storage tank - If time and rate of
energy generated by a solar energy system does not coincide with
energy needs, then thermal energy storage is required to store
the excess generated energy until it is needed. Ideally, the
storage capacity should be sufficient to store any excess energy
at a temperature most beneficial to both energy collection and
energy usage.
The size of the storage tank could be approximated according to
the specification of the load pattern as well as the estimated
average heating power output of the solar heating system. The
economic aspect should be also considered for the selection of
the tank size.
d) Sizing pipe line - Piping is required to
route and control the flow of heat transfer fluid between
various components of the solar subsystem. The objective of the
piping design is to accomplish all these functions with the best
compromise between minimum parasitic power requirements and
minimum capital costs. The pipe size should be determined
according to the flow rate required for the solar heating
system, maximum allowable flow velocity as well as economic
aspect.
e) Sizing Circulation Pumps - All solar water
heating systems, except the thermosyphon type contain a pump
set. The pump circulates the heat transfer fluid between the
collector and the storage tank. Pumps should circulate heat
transfer fluid at the design flow rate with minimum expenditure
of electrical energy. Analysis of the complete pipe work will
allow a total system head to be determined, describing variation
of the total pressure drop of the system with operating flow
rate. A suitable pump should provide the required flow rate at
the necessary head while operating at or near its best
efficiency.
The following table presents the typical design parameter ranges
for solar water heating systems.
| Collector
Flow Rate [gpm/ft²] |
0.05 -
0.07 |
| Collector
Slope |
(Latitude
+ 15 ° ) ± 15 ° |
| Collector
Orientation |
South ±
15 ° in Northern hemisphere |
| Heat
Exchanger Rate per Collector Area[W/K m²] |
40 - 80 |
| Storage
Capacity per Collector Area [Gallon/ ft²] |
2 - 4 |
|
| Overall
System Schematic |
The initial step of the
detail design effort should be the development of an overall
solar energy system schematic. When completely developed, the
schematic becomes a piping and instrumentation diagram and is
the overall guide for the detail solar energy system design
efforts. This diagram should graphically show or identify the
following information:
1. Overall solar energy system configuration and interfaces with
the loads and auxiliary energy sources
2. Major solar energy system components and their relative
locations (collectors, heat exchangers, thermal storage tank,
pumps)
3. Relative locations of the various pipe work components
(valves, vents, expansion tank, relief valves, etc.)
4. Relative location of required sensors and instrumentation. |
| System
Types |
The two commonly used
kinds of commercial solar heating systems are as follows: System
I is a solar heating system with an un-pressurized storage
tank. System II is the system with pressurized storage
tank. |
|
|
| System
I |
In System I, the
heat transfer fluid (ionized water) in the storage tank is
vented to atmosphere. All heating is done on a low pressure
basis (around atmosphere pressure). Heat is transferred to the
potable water by way of a heat exchange coil within the storage
tank.
The advantages of the system are that the entire system is at
low pressure so only potable water flows through the heat
exchange coil. As the stored potable water is at any time at a
minimum, this means isolated pockets of water cannot stagnate
and become Legionella breeding ground.
The disadvantages of the system are:
1) Higher collector loop pump capacity might be required to
overcome elevation head between collector and storage tank.
2) Initial cost of antifreeze fluid can be higher. |
|
|
| System
II |
In the system II, the
heat transfer fluid (neutral water) is stored in the heat
exchange coil as well as in the pipeline between the collector
and storage tank. Potable water in the tank is heated by the
coil. During the heating process pressure in the storage tank
will gradually be built up.
The advantages of the
system are:
1) The circulating pump needs only to overcome friction losses
in the pipeline because the collector loop remains full.
2) A full flow of pressurized hot water is assured at all times.
The disadvantages of
the system are:
1) Quality of service water must be good enough to prevent any
corrosion or excessive deposits in the storage tank.
2) A pressurized storage tank is required. |
|
E-Mail:
Thermo Technologies
Thermal Solar Collectors | Solar
Controller | Solar Tanks
9009 Mendenhall Court
Suite E
Columbia, Maryland 21045
Voice: (410) 997-0778
Fax: (410) 997-0779 |
