Solar Glossary and Key Terminology
Navigating the world of solar thermal energy involves understanding specific technical language. Below is a glossary of key terms to help you better understand solar hot water systems.
A
Aperture: The part of the collector through which light enters. For evacuated tubes, this refers to the cross-sectional surface area of the outer clear glass tube measured using the internal diameter, not the outside diameter. (Eg. 0.0548m x 1.72m = 0.094m2). 1.72m is the exposed length of the evacuated tube. Aperture is the most widely used surface area size used when quoting solar collector efficiency values.
Absorber: The part of the collector that actively absorbs the light rays. For evacuated tubes, this is defined as the cross-sectional area of the inner tube (selective coated) measured using the outside diameter. (Eg. 0.047 x 1.72m = 0.08m2). For evacuated tube collectors with reflective panels, the entire circumferential surface area of the inner tube is often used when calculating absorber area, as the reflective panel is designed to reflect light onto the underside of the evacuated tube. The Apricus AP solar collector does not use reflective panels, as they can become dirty over time reducing system performance, add significantly to wind and snow loading, and can trap leaves and other debris.
B
BTU: Stands for British Thermal Units. This is an imperial unit of measurement for heat widely used in the US and also in the UK. The conversion to the metric unit kWh is: 1 kWh = 3412 Btu, and for surface area values, 1kWh/m2/day = 314 Btu/ft2/day.
Active System: A solar thermal configuration that utilizes a circulation pump to move the heat transfer liquid through the collectors. Active systems offer flexible tank placement and allow for advanced freeze protection and smart digital controls. They are the standard for most modern residential and commercial installations.
Passive System: A system that relies on thermosiphoning action to slowly move water through the collector. This natural convection requires the storage tank to be placed physically above the collectors.
C
Circulation Pump: A component in active systems that moves the heat transfer fluid through the solar collectors and heat exchanger. Modern systems often use variable-speed DC pumps to optimize flow rates.
Concentrating Collector: A type of unglazed solar collector designed for high-temperature industrial applications. These require direct sunlight and complex tracking systems, making them impractical for standard residential use.
Controller: The digital brain of an active solar thermal system. It monitors temperature sensors and manages the circulation pump and backup heater. Controllers can be integrated with smart home energy monitors.
D
Domestic Hot Water (DHW): Hot water used inside the home for showering, bathing, and washing, distinct from water used for space heating or swimming pools.
E
Evacuated Tube Collector: A high-efficiency glazed collector consisting of parallel glass tubes with a vacuum seal to prevent heat loss. These excel in cold, variable, or overcast climates.
F
Flat Plate Collector: A widely used glazed collector featuring a dark, flat absorber plate in an insulated metal box with a tempered glass cover. Ideal for mild to warm climates.
Freeze Protection: A safety feature in active indirect systems where a closed-loop design is filled with a non-toxic antifreeze solution (glycol) to prevent the collectors from freezing in cold temperatures.
G
Glazed: Collectors that feature a protective glass cover to trap heat and reduce insulation loss. Flat plate and evacuated tube collectors are glazed.
H
Heat Exchanger: A device used in closed-loop systems where the antifreeze fluid transfers its heat to the domestic water supply without mixing with it.
I
Insolation: The amount of electromagnetic solar radiation incident on the surface of the earth. It is generally expressed in kWh/m2/day. Local insolation levels determine the required size of a solar collector.
P
Peak Efficiency: The maximum efficiency value advertised for a solar collector under ideal test conditions. For most thermal collectors, this value ranges from 60% to 80%.
PV (Photovoltaic): Technology that converts sunlight directly into electricity. While PV panels can power an electric water heater, the process is significantly less efficient than using solar thermal collectors for water heating.
R
Racking: The mounting frames used to secure solar collectors to a roof. Modern market trends favor pre-assembled and lightweight racking systems.
Residential Sizing: The process of determining the correct collector and tank size for a home. A basic rule of thumb is allocating one square meter of collector area for every one to two occupants.
Rule of Thumb: A basic, practical estimation method. For residential solar thermal applications, complex commercial modeling software is often not required, and rule-of-thumb sizing is sufficient.
S
Selective Absorber Coating: An advanced coating applied to the absorber plate or inner tube that maximizes the absorption of sunlight while minimizing heat loss.
Solar Thermal (ST): Technology that captures sunlight and converts it directly into heat for warming water or space heating.
Storage Tank: An insulated tank where heated water is kept until needed. Proper tank sizing and thermal layering (stratification) are essential for system performance.
Stratification: Also known as thermal layering. The natural phenomenon where hotter, lighter water rises to the top of the storage tank, while colder water remains at the bottom.
T
Thermosiphoning: The natural convection process used by passive solar systems to move water through the collectors without a pump.
Twin-coil Storage Tank: A type of storage tank used in active systems that features two heat exchanger coils to maximize thermal layering and solar input.
U
Unglazed: Collectors that do not have a glass cover or insulation. Low-temperature unglazed collectors are primarily used for swimming pool heating in warm climates.
W
Warranty: A guarantee on the system's lifespan. Quality glazed collectors often come with warranties of ten to fifteen years and can last twenty-five years or more with maintenance.
System Integration and Performance
Understanding these glossary terms is essential for grasping how a solar thermal system operates as a whole. A typical system incorporates a circulation pump, storage tank, controller, piping, and various valves. When pairing components, it is critical to match the collector type to your regional insolation and climate. For example, choosing an evacuated tube collector with a selective absorber coating is ideal for high-latitude regions with low winter insolation.
Furthermore, proper integration ensures that the high efficiency of the collectors is not lost in transit. Using variable-speed DC pumps and smart digital controllers optimizes the flow rate. Insulating the piping between the roof and the tank retains the thermal energy captured by the collectors.
Market Trends and Competitor Insights
Market analysis indicates a growing consumer preference for high-performance systems. Manufacturers focusing on lightweight materials and pre-assembled framing components see higher installation rates. There is also a noticeable trend toward hybridization, with forward-thinking competitors developing systems that integrate PV panels to power the circulation pumps and controllers of solar thermal arrays.
Frequently Asked Questions
What is the difference between aperture and absorber area?
Aperture refers to the area where light enters the collector (the outer glass tube), while the absorber is the part that actively absorbs the light rays (the inner coated tube). Aperture is most widely used when quoting efficiency values.
Why avoid reflective panels on evacuated tubes?
Reflective panels can become dirty over time, reducing system performance. They also add significantly to wind and snow loading on the roof and can trap leaves and other debris.
How do I convert BTUs to kWh?
The conversion is 1 kWh = 3412 Btu. For surface area values, 1 kWh/m2/day equals approximately 314 Btu/ft2/day.
Take Control of Your Energy Future
Equipped with this solar glossary and an understanding of system components, you are ready to evaluate your property's potential. Whether you are heating a backyard swimming pool with unglazed mats or powering your home's domestic hot water with high-efficiency evacuated tubes, the right knowledge makes all the difference. Consult with a local certified professional to start capturing free solar energy and secure your savings for the future.
