Adjust the collector area, irradiance, inlet water temperature and ambient temperature to calculate in real time the instantaneous heat output, daily energy, 200 L tank temperature rise and efficiency curve with the HWB equation.
Presets
Collector Parameters
Collector area A (m²)
m²
Heat removal factor FR
Optical efficiency τα
Overall heat loss coefficient UL (W/m²K)
W/m²K
Operating Conditions
Irradiance GT (W/m²)
W/m²
Inlet water temperature Tin (°C)
°C
Ambient temperature Tamb (°C)
°C
Results
While paused, moving a slider updates the results immediately.
What Is the Solar Water Heater Collector Simulator?
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What exactly does the "collector efficiency" calculated in this simulator represent?
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Roughly speaking, it is the fraction of the sun's light energy that is recovered as heat. For example, at an irradiance of 1000 W/m² and 50% efficiency, 500 W of heat per square metre is obtained. Move the "Optical efficiency τα" slider above: the higher it is, the higher the starting point of the efficiency curve (its left end).
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Why does the efficiency curve slope downwards? When I raised the "inlet water temperature" the efficiency dropped sharply.
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That is the effect of heat loss. When the water in the collector becomes much hotter than the surrounding air, heat escapes. In the formula, the term "$U_L(T_{in}-T_{amb})/G_T$" becomes large. In practice this shows up as high efficiency when cold water is fed in on a winter morning, and lower efficiency in the early afternoon once the water is already warm.
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I'm told that an evacuated-tube collector with a small "overall heat loss coefficient U" is better at high temperatures — how do I see that in this graph?
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Good catch. Choose "Evacuated tube" under "Presets" on the left. U_L drops from 4.0 to 1.0 W/m²K and the efficiency curve becomes much flatter — that is what "small U_L" means. This preset still gives 53.0% efficiency at an inlet temperature of 50°C, whereas the same inlet temperature with the standard flat plate (F_R 0.80, τα 0.75, U_L 4.0) gives 46.3%. In other words, the efficiency falls off less as the inlet temperature rises, which is an advantage when you want hot water at high temperature or in regions with low ambient temperatures.
Frequently Asked Questions
Standard flat plate (A 4 m², F_R 0.80, τα 0.75, U_L 4.0 W/m²K, G_T 700 W/m², T_in 20°C, T_amb 20°C), Evacuated tube (A 3 m², F_R 0.70, τα 0.80, U_L 1.0, T_in 50°C), Clear summer day (G_T 900 W/m², T_in 25°C, T_amb 32°C, F_R 0.82, τα 0.78, U_L 4.5) and Cloudy winter day (G_T 350 W/m², T_in 15°C, T_amb 5°C, otherwise as the standard flat plate). All are representative values for comparison, not the performance data of a specific product.
The daily energy assumes the set irradiance and ambient temperature stay constant and integrates the instantaneous heat output over 6 hours of peak sun. The 200 L tank rise is the temperature increase that this energy gives 200 L of water, using the specific heat of water (4.186 kJ/kg·K).
The optical efficiency τα, the overall heat loss coefficient U_L and the heat removal factor F_R can all be changed freely with the sliders (and numeric inputs) in the left panel. Derive each value from the manufacturer's catalogue as τα = F_Rτα / F_R and U_L = F_RU_L / F_R and enter them to obtain the efficiency curve and heat output of the real product.
In the HWB equation the heat-loss term becomes negative when the ambient temperature exceeds the inlet temperature, so the instantaneous heat output is larger than the ideal output from the optical efficiency alone. This represents the collector also absorbing heat from the surrounding air and reflects real behaviour.
Real-World Applications
Design of domestic hot-water systems:The required collector area is decided from the household size and the local irradiance. A typical use is to set winter conditions in the simulator (lower irradiance, lower ambient temperature) and check that enough hot water is obtained.
Agricultural and commercial hot-water demand:Effective where large volumes of low-temperature hot water are needed, such as maintaining water temperature in fish farms or hotel hot-water supply. Set a large collector area and simulate whether the daily energy obtained (MJ/day) meets the demand.
Comparing and selecting collectors:Enter the catalogue performance values (F_Rτα, F_RU_L) into the simulator and compare quantitatively how the efficiency differs under your own operating conditions (expected inlet temperature, local weather).
System safety checks (stagnation temperature):Estimate the maximum temperature the collector reaches (stagnation temperature) when the circulation pump stops. This is important for checking that it does not exceed the temperature rating of the collector and piping, and for taking overheating countermeasures.
Common Misconceptions and Points to Note
First, it is easy to overlook thatthe "irradiance" input is the value on the actual installation surface. The "in-plane irradiance" entered in the simulator differs from the horizontal irradiance. In Japan, for example, a surface tilted 30° to the south receives more irradiance than the horizontal in winter, when the sun is low, but about the same or slightly less at noon in summer, when the sun is high. In real design, the correct procedure is to derive the tilted-surface irradiance from a weather database.
Next,the "instantaneous heat output" refers to a single instant. For example, the 500 W/m² of heat calculated at 1000 W/m² irradiance and 50% efficiency corresponds to 0.5 kWh of heat energy if that condition lasts one hour. The daily energy is this instantaneous value integrated over a day. Since the same conditions do not persist from morning to evening, a step-by-step calculation using hourly irradiance and temperature data is more realistic.
Finally, note thatthe simulator's "efficiency" is the performance of the collector alone, not the hot-water efficiency of the whole system. For example, even if the collector produces 80°C water, it may arrive at the tank at 60°C because of large pipe losses. "Stopping losses", when collection stops because the tank is full, also lower the overall efficiency. This tool evaluates only the core collection performance; system design must also consider pump power, control logic and other factors.
Enter the collector area (m²). For a 4 m² flat-plate collector, for example, set "4"
Specify the irradiance (W/m²). About 800 W/m² on a south-facing normal surface on a clear day and 200 W/m² under cloud are useful guides
Set the inlet water temperature (°C). The results differ between starting from 15°C mains water and circulating 30°C water from a storage tank
Enter the heat removal factor F_R (0.7–0.85), the optical efficiency τα and the overall heat loss coefficient U_L (W/m²K) from the manufacturer's specification, and the instantaneous heat output is calculated with the HWB equation
Check the daily energy and the 200 L tank temperature rise to judge whether the system design is adequate
Worked Example
For a flat-plate collector with area A = 6 m², irradiance G_T = 700 W/m², inlet temperature T_in = 20°C, ambient T_amb = 25°C, F_R = 0.75, optical efficiency τα = 0.75 and U_L = 6 W/m²K, the efficiency is η = F_R[τα − U_L(T_in − T_amb)/G_T] = 0.75×[0.75 − 6×(20 − 25)/700] ≈ 0.595 (59.5%). The instantaneous heat output is Q = η·A·G_T = 0.595×6×700 ≈ 2.50 kW. With this tool's 6-hour peak-sun conversion that is about 15.0 kWh/day, and the rise of a 200 L tank (specific heat 4.186 kJ/kg°C) is about 64°C. The stagnation temperature is estimated as T_stag = T_amb + (F_R·τα/U_L)·G_T ≈ 90.6°C (shown rounded as 91°C).
Practical Notes
In the rainy season (June) and in regions with much cloud, the irradiance falls below 300 W/m² and the heat output drops sharply, so size the auxiliary heat source adequately
The irradiance G_T is the value incident on the collector surface. If the weather data give only horizontal irradiance, conversion to the tilt and orientation of the installation surface is needed; a south-facing tilted surface receives more than the horizontal in winter and about the same or slightly less in summer
Efficiency falls linearly as the inlet temperature rises (under the standard flat-plate conditions: 60.0% at T_in 20°C, 48.6% at 45°C, 32.6% at 80°C). The heat output approaches zero as the stagnation temperature (125°C under the same conditions) is approached, so pump-stop decisions and overheating protection matter at high temperatures
The intercept F_Rτα and slope F_RU_L of the efficiency curve change over the years through cover soiling, degradation of the selective absorber and damp insulation. Once measured values after installation are available, enter them in the sliders and recalculate