Heating systems with heat accumulators

Recommendation points

• What is a heat storage device
• What does the installation of a heat accumulator give
• Selection criteria for a buffer tank
• Calculation of the heat accumulator
• Features of installation and operation

There is a way to bring the convenience of using a solid fuel boiler closer to the comfort that heating on natural gas provides. The heat accumulator not only reduces the frequency of kindling, but also provides significant savings in electric heating with a multi-tariff payment system.

What is a heat storage device

The task of conserving thermal energy is one of the most pressing problems in the world energy sector. First of all, its solution is required to optimize the operation of solar energy conversion systems, but the same kind of issue is also in ordinary households. Some owners of country houses are not attracted by the idea of ​​becoming a fireman, while those who use electricity for heating are primarily concerned about cost reduction.

All these tasks can be solved by installing a heat accumulator, which accumulates energy in the mode of active operation of the heater, and then transfers it to the system for a long time. To avoid confusion, it should be noted right away that heat storage devices are of two types:

1. Magnesite heat accumulators installed in electric heating devices.
2. Buffer tanks with water – a natural substance with the highest heat capacity.

The first type of heat storage has been known for about half a century and serves as a replacement for electric convectors where multi-tariff payment for electricity is relevant. The stored heat of the magnesite shell is capable of giving from 8 to 12 hours, heating the house during the day without consuming electricity. The main disadvantage of such heat accumulators is the need to accurately calculate the thermal capacity individually for each room, taking into account the inertia of building structures, heat loss and air volume. Errors in calculations lead to a violation of the climatic regime and, as a consequence, the absolute inexpediency of using heat accumulators of this type.

The buffer tank is a water tank integrated into the hydraulic heating system. The heating of the buffer coolant can be carried out directly – such tanks are called single-circuit, or through a heat exchanger, that is, the coolants in different circuits are isolated from each other. The multifunctionality of the water storage allows it to be used both for uniform heat transfer in transient modes and for energy storage during the period of the night tariff. It is this type of heat accumulators that is most interesting for the Russian consumer, and we will dwell on it in detail..

What does the installation of a heat accumulator give

The heat accumulator cannot be classified as a budget heating system upgrade. The average cost starts from 120-140 thousand rubles. and can reach half a million, and the specified price range is relevant for systems that provide heat to housing with an area of ​​up to 150 m². A natural question arises: what is the justification for the purchase of such an expensive equipment??

To understand the feasibility of installing a heat storage device, you need to understand well the specifics of its operation. First of all, it is worth remembering that it does not provide tangible advantages in tandem with a boiler that runs on natural gas, fuel oil, that is, with automatic fuel supply. It makes sense to install a heat accumulator either with electric or solid fuel units with manual loading, as well as in conjunction with solar collectors. In this order, a number of advantages can be distinguished:

1. Improving the operating comfort of a solid fuel boiler – the number of kindling can be reduced to one per day, and this is true both for the off-season and during the coldest five-day period.
2. Increased savings: operating at maximum power, the solid fuel boiler does not lose heat along with the combustion products. Low temperature water circulates in the coil all the time, effectively absorbing the generated energy.
3. Extending the service life of the boiler due to overheating protection. In the mode of active flame combustion, the heat exchanger remains dry all the time – due to the absence of condensate, the pollution of the smoke channels with tar is significantly slowed down and the effect on the coil metal of acids formed when smoke comes into contact with moisture is excluded.
4. If the system is powered by a solar collector, the heat accumulator is indispensable, because it is thanks to it that it is possible to heat the habitable volume not only during the day, but also at night.
5. The design features of the heat accumulator make it as easy as possible to implement a DHW system, including with recirculation.

Selection criteria for a buffer tank

As already mentioned, the price range for heat storage devices of identical capacity is very high. To make the right choice, you need to know what determines the quality and durability of equipment. The material of the container body is of prime importance:

• Carbon steel can be used in systems that do not have a regular water make-up, which means that metal elements are not constantly exposed to dissolved oxygen.
• Stainless steel has anti-corrosion properties, such a container is completely incapable of causing sludge formation in the system and is considered the most advanced type of heat storage, but it is more expensive than others..
• A metal case with a polymer coating – in this case, the corrosion resistance of the base material of the tank is not critical, because the protective shell guarantees the purity of the liquid. However, you need to be careful: some polymer compounds can react with special heat carriers, in addition, the thickness of the metal shell must be high enough to withstand mechanical stress.

Heat accumulator designs: 1 – direct connection of circuits; 2 – with one heat exchanger; 3 – with two heat exchangers

Buffer tanks made entirely of plastic should not be considered seriously. Such storage devices are not able to withstand the load from the liquid expanding when heated; moreover, at high temperatures, many polymeric materials lose their properties..

Not only the material itself is important, but also the structural features of the case. So, the best option is a cylindrical shape with a one-piece stamped spherical bottom. Imported equipment, among other things, is characterized by a higher quality of welding, is tested by increased hydraulic pressure before delivery of products to the market. The complexity of the equipment also plays a role: the simplest accumulators are included in the system directly, but if a special heat carrier is used in heating, heat transfer is carried out according to the scheme: boiler circuit – storage – heating circuit through heat exchangers. In this case, protection against water freezing in the storage tank is implemented by installing an electric heater, which maintains a positive water temperature in idle mode. Energy consumption is negligible thanks to the continuous thermal insulation of the buffer tank.

Another nuance is the type and quality of thermal insulation, which also directly affects the cost. Budget models are dressed in a casing made of mineral wool and expanded polystyrene – not the most ideal materials in terms of durability and fire safety. More advanced models have polyurethane foam thermal insulation, and the most expensive ones are made of polyisocyanurate. These materials retain heat better than others; under operating conditions of the drive they serve for tens, if not hundreds of years, while they do not pose a threat of fire development..

The following optional additions can also affect the cost of the heat storage device:

• Rigid shell to improve aesthetics and protect thermal insulation from mechanical damage.
• The presence of an electric heating element or a branch pipe for its installation.
• Additional taps from the DHW coil or additional heat exchangers.
• Built-in thermometers, pressure gauges, capsules for installing electronic sensors.

Calculation of the heat accumulator

In fact, the only parameter of the heat accumulator that needs to be determined for proper integration into the heating system is its capacity. The calculation is carried out according to a simple scheme: the time for which the system cools down to an uncomfortable value in the absence of energy supply is determined. You need to divide the desired autonomous operation time by this value, then apply the resulting coefficient to the own capacity of the heating system.

For example, if, with a system volume of 100 liters, cooling occurs in an hour, and heat is required for 12 hours from one kindling, the total amount of heat carrier should be 1.2 tons, respectively, the required buffer capacity is 1100 liters. At the same time, one should take into account the increasing heat losses of the system, which do not exceed 10% even in the most budgetary storage options..

An approximate formula for the selection of the volume of the heat accumulator:

m = (P? t) / (c? T)

Where: m– volume
P– boiler power, W
?– boiler efficiency, 0.98%
t– heating time, h
c– specific heat capacity of water, Wc / kg K
?T– temperature delta, K

When operating from an electric heating unit, the capacity of the buffer tank is calculated according to the boiler power and the permissible electrical connection power of the facility. Knowing the amount of energy required to heat water with a given temperature difference, it is necessary to select such a volume so that the buffer tank is completely warmed up during the low tariff. For example, in order to heat one liter of water with a delta of 80 ° C, 93 W / h of electricity is required with the system efficiency equal to unity, while the heating duration will be about three minutes. In order not to get confused in the intricacies of formulas, you can use a convenient online calculator.

Another aspect of the system sizing is related to the boiler output. It should be about twice as high as needed for a system without a heat accumulator. Firstly, this will allow increasing the fuel loading and firing up in the high power mode, which means that you will have to throw up firewood less often when the system is heating. Secondly, with an increase in the boiler power, the surface area of ​​the heat exchanger increases, respectively, the amount of heat not absorbed by it will be lower.

Features of installation and operation

The storage tank piping scheme does not depend on the presence and number of intermediate heat exchangers. When connecting a heat accumulator, it is customary to separate the boiler and radiator circuits. Each of them requires the installation of three-way valves:

• A mixing valve on the boiler circuit is necessary so that the boiler does not operate in condensing mode while the entire volume of water in the heat accumulator is heating. Mixing hot water from the supply to the boiler return allows you to maintain the temperature of the heat exchanger at a level at which the soot will not be moistened.
• A control valve on the radiator circuit allows you to regulate the temperature of the water supplied to the heaters. Firstly, this allows you to extend the battery life, in addition, it becomes possible to comply with temperature restrictions for systems with plastic pipes or underfloor heating without additional technical means.

Wiring diagram for a solid fuel boiler with a heat accumulator: 1 – solid fuel boiler; 2 – thermostat; 3 – security group; 4 – air separator; 5 – circulation pump; 6 – check valve; 7 – buffer tank (heat accumulator); 8 – three-way valve; 9 – automation; 10 – heating circuit; 11 – surface-mounted temperature sensor; 12 – expansion tank; 13 – dry running sensor; 14 – make-up valve

When installing a heat accumulator, two forced circulation circuits with separate pumps are formed. At the same time, hot water is supplied to the heating devices as soon as possible after starting the boiler, since the coolant is taken from the upper part of the tank, where the liquid with the highest temperature is sorbed. It is important not to allow the installation of pumps after the branch to feed the valves, so that if they are completely closed, the circulation of the system does not stop. At the same time, there is no difference in whether the pump is installed on the supply or return branch, there is no.

When operating a solid fuel boiler with a buffer tank, it must not be allowed to operate in condensing mode. In systems without a heat accumulator, it is customary to limit the supply of oxygen, so you can extend the burning time of the bookmark. In the presence of a buffer tank in the heating mode, the boiler operates in the flame combustion mode all the time, which is possible due to the significant volume of the coolant, which is capable of absorbing a colossal amount of thermal energy. To avoid the formation of condensation, the three-way mixing valve must be set to a temperature not lower than 60 ° C. Perhaps due to a decrease in the temperature delta, the heat transfer will not be so active, however, this is the only way to exclude overmoistening of the heat exchanger, which will facilitate cleaning from soot and extend the service life of the heating unit.