Does A Solar Water Heater Work At Night: How does it work?
Before installing a solar water heating system, you must be wondering if the solar water heater can give you hot water at night or not! If yes, then how does it work? This article will clear all the doubts you have. I am a physics graduate, and I have been studying the theoretical aspect of solar water heaters for a long time now. And I noticed the technological development of solar water heaters has been progressive over the last few decades. And the exciting thing is, nowadays you can even store hot water for several days, which was not feasible in the past. Don’t worry! We’ll dive deep into the topic. Before that, let’s start with a brief answer: Does a solar water heater work at night?
Although solar collectors of a solar water heating system can not work at night without solar radiation, solar water heater storage can store the hot water generated during the daytime and supply it to you at night. Also, almost all the active solar water heating systems come with backup power, where you can heat the water at night using grid power or even with solar (PV) battery storage.
Next, we’ll try to understand the above answer in great detail! Please keep reading to know how they work during the night.
Why don’t solar collectors work at night?
We need to understand the working principle of a solar collector, which is extremely simple. Solar water heaters are a great example of how we can engineer a great product with basic physics principles. In quick words, solar water heaters work on the principle of “heat transfer.” If they don’t receive the heat from the Sun, they can not operate.
Working Principle
First, solar collectors absorb the heat from solar radiation and then transfer it to the rest of the system. Here in solar water heating systems, solar collectors are basically two types:
1. Flat plate collector
2. Evacuated tube collector.
You’ll mostly see evacuated tube collectors in most of the system.
Next, we run a heat transfer fluid through the system. Thus the absorbed heat by the collectors transfers to the heat transfer fluid. And Then, heat transfer fluid flows into the heat exchanger to heat the cold water by the method of convection.
During the nighttime, as you can not get the heat from solar radiation, your solar collectors do not operate to heat up the heat transfer fluid.
But, as I mentioned earlier, we have been going through many developments on making the solar water heating system efficient. And, right now, we can expect to get hot water from the system at any time, whenever we need it. In the following sections, we’ll understand how it is possible.
How can a solar water heater system still provide hot water at night?
We’ll discuss the three main things that can ensure you get hot water at night.
1. Storage Tank
A storage tank is an essential component of the solar water heating system. Usually, we do not use the hot water made by the system immediately. Sometimes, we shower in the early morning, and sometimes even at night! So we need a space to store the hot water until we use it. That’s why the storage tank is essential to preserve hot water efficiently.
Many manufacturers claim their system can keep the water hot for 72-80 hours. I recently asked Suntask (a regional solar water heater manufacturer in China) on Twitter if we can get the hot water at night, and they said the tanks have a preservation time of 72-80 hours.
Although this preservation time varies a lot depending on the manufacturers and climate of your area, I can confidently say that with good insulation, a tank can preserve hot water for at least a night!
Insulation of storage tank
Insulation of the storage tank is the most important factor why it can retain the heat for a long time. However, some models do not have good storage tank insulation.
In that case, I recommend wrapping the tank with an insulation material that can withstand the huge temperature difference. Make sure it is at least rated 200° C or above. Usually, foam, fiberglass, and thermoplastic insulation can be great for avoiding any heat loss. Also, it is crucial to prevent any water leakages from the tank.
If unsure how to take these steps, contact a plumber.
Proper sizing of a storage tank
Proper sizing is vital if you want to store hot water for a long time. You must check your usage and calculate the size of the tank and system accordingly. If you don’t have enough storage space, you can not get enough hot water supply at night.
2. Insulation of the whole system
Insulation of the whole system is crucial to make the system efficient in supplying hot water for a long time! There are three components in a solar water heating system that we need to insulate.
- Solar Collectors: To minimize heat loss from the side and back to the surrounding.
- Pipelines: To reduce heat loss to the surroundings while carrying the hot water.
- Storage Tank: To minimize heat loss to the ambient while storing the water for an extended period.
We already discussed the storage tank. Now, if we can insulate the pipelines and collectors, we don’t have to worry about preserving hot water for the night.
Requirements for good insulating materials for a solar water heating system
- Extremely low thermal conductivity.
- Low water absorption.
- Stable in high-temperature variations.
- It does not degrade with harsh weather conditions.
- Cost-effective and eco-friendly.
3. External power backup
External backup is very common these days to meet the excessive requirements of hot water at night. Especially in colder regions, an external electric backup is beneficial. Now you might think our aim in installing a solar water heating system was to ensure we are going green and saving the cost. So why would we use the power from the grid?
Well, this hybrid model is still better than a traditional electric water heater which uses electricity directly from the grid. It’s proven to be more sustainable if you compare the energy usage and total cost (including monthly electricity bill).
To implement a power backup, you can either make a separate tank connecting to the main solar water storage tank or use the same storage tank. But make sure to get help from a professional engineer while installing the power back up.
Can you use a PV system as a power backup?
You can do that if you already have installed PV panels. However, it will increase the whole system’s cost by a significant amount. Additionally, maintenance and battery storage costs are also a must consideration.
So these were some definite reasons and explanations for why and how we can get hot water from a solar water heater at night. However, here is a twist! How about getting hot water on a freezing night? Can the storage tank keep the water warm enough? In the next section, we are going to discuss that.
Can we get hot water on a freezing night?
If we use a hybrid solar water system with an electric backup, then yes, always! However, without an electric power backup, I have noticed in many systems that water storage tanks struggle to keep the water hot during freezing nights. And that is primarily because of incorrect installation, leakages, and weak insulation. Here we have to accept that insulation can never be 100% perfect if you want an economical solar water heater, and even the costlier systems can not maintain it.
Conclusion
To conclude, solar collectors can not operate without the Sun. But, the system can store enough hot water for the whole day and night if you have proper sizing of the solar tank. Next, make sure to insulate the system well, fix leakages, and use power backup to get a seamless hot water supply.
