Solar collectors

Solar collectors

Solar collectors provide hot water in a sustainable way. Sunlight is infinite and that does not apply to the sungas supply. On average, that saves about 50% on your energy consumption of your hot shower and tap water. But what does a solar water heater with collectors cost and what is the payback period? And what types of collectors are there?

Solar collectors come in many different designs based on many different techniques. We distinguish them in the following types:

  • Flat plate solar collectors
  • Vacuum tube collectors
  • Hot air collector


Most common are the flat plate collectors and the vacuum tubes version, this is also called heatpipes. Both are preferably placed on the roof at an angle, in order to convert sunlight into hot water and then bring it to the boiler.

Flat plate solar collectors

The flat plate solar collectors work on the principle of a black plate covered with a glass plate. The advantage of this flat plate solar collector is its black color which often remotely resembles a solar panel. Normally the size of a flat plate collector is different from that of a solar panel although there are now collectors with the same dimensions. Aesthetically, therefore, a nice choice.



Vacuum tube collectors

These solar collectors work with a number of glass pipes, made of double glass. This double glass has been vacuum-sealed and placed behind these glass tubes in a so-called parabolic mirror that deflects the diagonally incident solar rays into the tube, thereby directing more energy towards the fluid in the tube.

This makes this collector less sensitive to the correct angle of inclination on the roof. A special coating ensures that the heat is retained as long as possible. This system is up to 40% more efficient than the flat plate collector and that is especially important in winter when conditions are not ideal. Necessary if you want to use the solar collectors to heat the house. After all, in the winter you need the heat the most.

Hot air solar collectors

The operation of hot air solar collectors is best compared to the time-honored thermos. They consist of a glass or metal Thermofront wall with a cavity behind it containing air. Fresh outside air is drawn in through the metal wall and this air is heated by the sun behind the wall. This is often done with active ventilation. This pre-heated air can ideally be piped to an existing air treatment system and, failing that, distributed through the building via specially constructed ducts. This technique is mainly seen in non-residential construction. Because of the considerable modifications required, you will find these hot air collectors particularly in new construction. Hot air collectors have a very high efficiency.

Who at home as a handyman wants to use this principle, can orientate on

From collectors to solar water heater

The solar collectors absorb sunlight and release this heat to the special solar fluid in the collector. This liquid goes to a so-called reactor vessel or solar water heater and there gives the heat with the help of a heat exchanger back to the tap water. On very sunny days, the water in the collector can become as hot as 90 degrees Celsius. After the heat is released to the tap water, the cooled water is pumped back to the collector to be heated again. In the summer months the solar fluid becomes sufficiently warm but in the winter a reheater is needed to get the boiler temperature to the minimum temperature of 60 degrees Celsius.

This temperature of 60 degrees Celsius is not only the right temperature for the boiler water to heat the house, but it is also the minimum temperature at which the legionella bacteria have no chance. The boiler can often take on this role of reheater if it is suitable for it. Make sure you use a combi boiler with the NZ label, which is specially designed for this. Combination boilers with this label are suitable because they are protected against overheating up to 85 degrees Celsius. If possible, install the storage tank near the combi boiler.

How many solar collectors per 100 gallons of water?

Water consumption is the starting point for determining how big the storage tank should be. And water use is determined by the number of occupants, the size of the property and, of course, the behavior of the occupants. Households with a pool logically need a much larger tank.

For an average family of 4 people, one often uses a storage tank of 150 – 250 liters. When the solar collectors are perfectly oriented to the south, one calculates 1.3 tubes per 10 liters. In case the roof is more to the west, the following calculation is closer to the truth.  You should then count on 40 vacuum tubes per 250-litre drum and 48 vacuum tubes for a 300-litre drum. 48 vacuum tubes means 2 collectors of 24 tubes each.

The storage tank

The storage tank is made of a stainless steel inner wall and a stainless steel outer wall. With between a highly insulating PUR foam layer so the water stays warm for a long time. The energy label of the storage tank shows how big the heat loss is. Often there are 2 heat exchangers in the boiler. The lower one is connected to the solar collector and the upper one to the central heating boiler for the after-heating.

In addition to the storage tank, combi boiler and solar collectors, a control unit is needed with temperature sensors, control panel and built-in software to customize the system for optimal efficiency.

Green declaration for solar collector systems

For operators of solar collector systems (including owners / residents of residential buildings) is possible with a so-called green certificate, an advantageous loan for investments in solar collector systems, among others. This loan can also be used for the matching heat storage. Owner/occupants who use collectors for tap water and space heating can also apply for a green certificate. This application can be made by a bank with a green fund on your behalf at the RVO, part of the Ministry of Infrastructure and Water Management. This green certificate is valid for an average of 10 years. The minimum investment for a green certificate is € 25,000.

Do you need a permit to install collectors?

For the installation of solar collectors the same requirements apply as those imposed by the government for the installation of solar panels. As long as the collectors do not extend beyond the roof edge and it does not concern a listed building, no permit is required to install them. Buildings that fall under protected town or village views generally require a permit. Check with your municipality.

Subsidy for solar collectors

Under certain conditions you can get a subsidy for an investment in a solar water heater. Since January 1, 2020, this is no longer possible for installations in new buildings, but it is possible for existing buildings. The amount of the subsidy is between € 750 and € 2,000. You need to apply for the subsidy within 6 months after installation, submitting the proof of payment. For more information see:

Cost and payback time solar collectors

The cost of a solar water heater with solar collectors and storage tank depends on the use. For a 4-person household, a collector of 3.5 m2 and a storage tank of 150 liters are sufficient. In the summer this system provides all your hot water needs. In the winter this is not enough and the high efficiency boiler or heat pump heats the water further to the correct boiler temperature of about 60 degrees.

Solar collectors, the costs and savings

The complete working installation requires an investment of € 3,300, – including installation without subsidy. (If you subtract the subsidy of € 1,100, – remains a net investment of € 2,200, -. With this solar water heater you save about 180 m3 of gas per year, which is a saving of € 150 per year and 330 kg of CO2.

Now the pump in this set also uses electricity to transport the hot water to and from the storage tank. Therefore, if you choose a pump with energy label A that consumes about 40 kWh, the costs will be about € 10,- per year. In addition, count on € 50,- per 5 years for maintenance in case of a solar water heater with drain back system. This leaves a net saving of € 130,- per year.

If we disregard the interest costs, the payback period is about 17 years. If you have above-average water use, the payback period will be more favorable. And with rising gas prices you will also get the investment out of it sooner. With an annual increase of 7% for the gas price including taxes, the payback period is 7 to 9 years.

Difference between payback period for solar panels and solar collectors

If you have enough roof space, solar panels are an investment that you can earn back more quickly. Usually in about 7 years. However, if you have a small roof area or you already have solar panels and you deliver the maximum in the salder scheme back then a solar water heater to consider. Also, the solar collector is somewhat less sensitive to shadows on the roof. With a little bit of shadow a solar panel is already quite strong in performance, which is much less the case for the solar collector.

Heat Battery

TNO, in collaboration with partners in the SSUSG consortium (Sustainable Selfsupporting Urban Smart Grid), has been researching a compact, lossless heat battery for the built environment. They started working on this in 2016 and by 2020 it looks like the heat battery will find its way into the energy system of the future. TNO’er answers some questions.

What is a heat battery?

“Just as an ordinary battery stores electricity, the heat battery stores heat. For example, summer heat can be stored to dispel winter cold. TNO is working in the international consortium MERTIS to develop a prototype of a heat battery suitable for homes.” Says Huub Keizers, program manager for energy in the built environment.

The current methods of heat storage are not optimal. A lot of heat is lost during storage and transport. TNO has now succeeded in reducing this to 0.35 GJ/m3, which is a world record. An average house uses 32 GJ/m3 per day. The result of the thermal battery: large amounts of heat can be stored and released losslessly for a long period of time.

How does it work?

Huub Keizers explains below how a heat battery works:

“The heart of the heat battery is a large container filled with salt. For the enthusiasts: TNO’s system involves sodium sulfide, a material that is cheap and widely available. Charging and discharging the battery is based on hydration. The heat entering the battery dries the salt. When water is added later, the heat is released again. This is called thermochemical storage. Collecting the heat can be done with solar collectors, like the ones people already have on their houses. The heat released from the battery can be used to heat water in a boiler. The hot water can then be used for central heating or as hot water.”

The home heat battery relies on two components, water and salt (hydrate). When the heat battery is charged, a drying reaction takes place. Heat is released in this process. The prototype developed in cooperation with partners from SSUSG (Sustainable Selfsupporting Urban Smart Grid) consists of two insulated modules for the salt and for the evaporation of water. In this way, the charging and discharging of heat can be done in a compact device that easily fits into a home.

The battery developed by TNO can store heat for long and short periods of time. For example, it can store energy in the summer and provide hot water for the house in the winter. It has been predicted by TNO that if the battery is charged and used once a month, it will have a life span of at least 20 years.

Why is storage necessary?

Nobody can predict in detail how we will ensure that the Netherlands is climate neutral in the future. Renewable energy sources produce energy irregularly. Peaks in the supply make it necessary to store energy. The heat battery can take advantage of this and provide buildings with heat at peaks in the demand for sustainably produced energy.

Huub Keizers: “We are working towards a future in which houses no longer consume energy, but actually produce energy. This will involve well-insulated, pleasantly liveable and healthy houses that have systems for generating energy. These include solar panels for electricity and solar collectors for heat. To make a house truly energy-neutral, energy storage is essential. Even across the seasons. On sunny summer days production is high and consumption low, on dark winter days the opposite is true. With a thermal battery, the surplus from the summer can be used in the winter.”

What is TNO doing?

“TNO is one of the participants in the European cooperation project MERITS. The aim of the project is to demonstrate the technical feasibility of the heat battery. In addition to scientists and researchers from institutions such as TNO, industrial partners and SMEs are also involved. The project team has built a prototype, which is now coming to the Netherlands. The technology works, now it’s a matter of getting it ready for the market.

In the coming years TNO researchers will be working on refining the design. There is an important step to make the system more compact. For example, by increasing the energy density of the salt and refining the supply and removal of heat and moisture. The better you do, the smaller the system becomes. Longevity and stability are also important: the module must be able to charge and discharge hundreds of times without problems. The main goal now is to make the heat battery smaller, preferably ten times as compact. With the current efficiency, the heat battery would still be too large for the home. We want to move towards a maximum of three cubic meters, so that the battery can fit in a basement or crawl space, for example.”

When will the battery be for sale?

In 2016 a prediction was made, Huub Keizers substantiates this; “We hope to have a product on the market for ‘early adopters’ in six years. However, it is too early to talk about the payback period of the heat battery. That depends on so many factors. But it is clear that the price must be such that it is interesting for homeowners. TNO’s further development of the heat battery is taking place within the Multi-Year Plan Compact Conversion and Storage and the European CREATE project, among others. TNP is putting a lot of energy into the heat battery, so that it can later be used in style as an updated technology for the building and energy market.”

In recent years TNO has invested heavily in compact heat storage by means of heat batteries. As a result, TNO has a lot of knowledge of active materials, components, reactors, and systems. Various projects are continuously running in this area.

In 2020, two concepts for a compact heat battery based on salt hydrates have been developed. a concept for based on a vacuum reactor. and one based on a closed loop.  Both concepts have now reached a Technology Readiness Leven 6 to 7, and are thus ready for the market.

Paris proof comitment

During the Dutch Green Building Week (DGBW) there have been 3 TNO-Webinars. After signing the Paris Proof Commitment of the Dutch Green Building Council, TNO is using its commitment to achieve a sustainable built environment.

Managing director Machteld de Kroon of TNO Construction, Infrastructure and Martiem says: “We signed this commitment because making the built environment Paris Proof is a goal we have been working on for years with our partners. We do this through research, development of innovations and knowledge transfer, for example in the field of heat recovery, storage and energy performance modeling.”

TNO is not only signing on as a knowledge partner of the Prais Proof Commitment: “TNO itself also has a considerable real estate portfolio. When investing in our own real estate, it is also about practice what you preach.”

Thermochemical principle

A heat battery consists of two parts, components, namely water and salt hydrate. When the water vapor and salt are combined, the water binds to the salt. The salt then changes to a new crystal form, heat is released in the process. This is called Thermochemical Principle.

To get the whole thing back to two components, the following happens. Heat is used to get the water off the crystal form. Basically, it is heat that is stored and as long as the water and salt are separated, the heat remains stored. In this way, a lot of heat can be stored, without heat loss, which in turn is needed to store it for a long period of time.

TNO has developed a new device to prevent the salt particle from losing its cohesion, tearing or falling apart. The new device uses the so-called “closed-loop principle”.