Solar Hot Water Basic Knowledge
Understand Hot Water Heating System in the Home
1kWh of heat energy can heat about 30 Litres / 7.5 US gallons of water up from cold to showering temperature.
A household of 5 people will use between 10-15kWh of energy each day to heat water (not including space heating).
In the winter more energy is needed to heat water as the cold water is colder, and people often have longer and hotter showers than in the summer.
Water is normally heated by electricity or gas, but solar can be used to offset between 50-90% of this energy.
Calculate Home Energy Consumption for Water Heating
A simple formula to estimate the energy required to heat a volume of water is :
E = C*M*DeltaT/PR/860
Where E = energy in kWh
C = Specific heat of water – 4.187 kJ/kgK, or 1,163 Wh/kg°C
M = weight of water to heat /kg
deltaT = Th-Tc /°C (Th = temperature of hot water; Tc = temperature of input cold water)
PR = performance ratio (it includes losses of heat through pipes and tank), default value = 0.9
860 = energy conversion from Kcal to kWh
Based on your hot water consumption and required hot water temperature, now you can calculate how much energy you need to provide that much hot water.
Storing Hot Water – Thermal Layers in Tank
The most common hot water system involves heating and then storing hot water in a tank.
The tank is always being gradually heated up from top to the bottom.
The hot water is lighter which goes to the top of the tank, the cold water is heavier and sits at the bottom of the tank. Therefore, it’s realistic to see a tank that has the bottom of cold water and top of hot water…this is referred to as thermal layering, or stratification.
Hot Water Usage from Tank
When hot water is used, it is drawn from the top of the tank, where the water is the hottest, and fresh cold water is delivered into the bottom of the tank.
Eventually after enough hot water usage the water suddenly turns cold, which is something everybody has probably experienced!
Vertical tanks are designed to enhance thermal layering to maximise hot water supply by reducing turbulence when the cold water enters the tank.
Solar Tank Design
The performance of a solar collector is highest when heating cold water.
The efficiency of solar collector drops when the water temperature increases.
For this reason thermal layering is important in a solar storage tank, ensuring that solar is always heating the coldest water possible.
Given that the coldest water is in the bottom of the tank, the solar heating is always in the bottom area, either direct flow or with a coil heat exchanger.
Retrofit Normal Tank to a Solar Tank
Case One:
If you don’t want to add a solar tank, you can simply retrofit the existing hot water tank as below piping layout.
However, the below retrofitting cannot avoid the competition between solar and existing conventional energy sources.
Case Two:
Ideally a dedicated solar tank should be used which then feeds a primary tank with solar pre-heated water.
You’d like to pay cost for a solar tank so as to optimize using of solar energy.
This format does not require any major changes to the plumbing layout. The cold supply is fed into the solar tank, and the solar tank outlet feeds the cold inlet of the main tank.
Having a dedicated volume of water for the solar to heat eliminates competition from the existing energy source.
How Much Solar Hot Water?
So how much hot water will a solar collector give you? To answer this question fairly, one variable must be fixed, either temperature rise or volume of water.
The following examples use a nominal heat value of 10kWh. The actual output of the collector depends on the size, time of the year and other factors.
Set Temperature Rise
If raising cold water by 30°C / 54°F to a suitable showering temperature:
10kWh can provide about 285 Litres / 75 US gallons of hot water.
Set Volume
Temperature rise for 200 Litres / 53 US gallons of cold (15°C / 60°F) water:
10kWh will heat the water to about 58°C / 136°F
So a much better question to ask is “How much showering temperature hot water will this collector give me in summer?”. That is a question that a solar professional should be able to answer confidently based on a specific collector model, system configuration and your local climate.