3 Brine / water heat pumps

3.1 Ground heat source

Possibility of use

• monovalent

• monoenergetic

• bivalent (alternative, parallel)

• bivalent regenerative

3.1.1 Dimensioning information - ground heat source

The geothermal heat exchanger, which serves as a heat source for the brine / water heat pump, must be designed for the cooling capacity of the heat pump. This can be calculated from the heating power minus the electrical input power of the heat pump in the design point.

The basic rule for the heat source is that the power Q transferred to the heat pump's evaporator₀ must make permanently available. The following applies:

Evaporator output Q₀ (kW_th) = Heating capacity Q_C. (kW_th) - electrical power consumption of the compressor P_el (kW_el)

When replacing an old heat pump with a newer model, the performance of the geothermal heat exchanger must therefore be checked and, if necessary, adjusted to the new cooling capacity. Here, the minimum brine temperatures and the running times of the past heating periods provide important information about the heat source.

• Brine temperatures are well below 0 ° C over a longer period of time.
  => The heat source may not be able to guarantee the higher extraction capacity of a more efficient heat pump. The installation of a second heat generator, e.g. a heating element, is recommended

• The heat pump has only a few hours of full annual use
  => The heat pump seems to be oversized. Replacing it with a heat pump with a lower heating capacity leads to longer running times, lower peak extraction rates and thus more efficient operation. 

The heat transport in the ground takes place almost exclusively through thermal conduction, whereby the thermal conductivity increases with increasing water content. Just like the thermal conductivity, the heat storage capacity is largely determined by the water content of the soil. The icing of the water contained leads to a significant increase in the amount of energy that can be recovered, since the latent heat of the water is very high at approx. 0.09 kWh / kg. For optimal use of the ground, icing around the pipe coils laid in the ground is therefore not disadvantageous.

Dimensioning of the brine circulation pump
The brine volume flow depends on the performance of the heat pump and is conveyed by the brine circulation pump. The circulating pump is to be dimensioned in such a way that a mass flow corresponding to the evaporator output is conveyed. Depending on the output, the mass flow should be selected so large that a temperature spread across the evaporator of 2 - 3 Kelvin is set at the lowest heat source temperature. At higher brine temperatures (e.g. summer operation / hot water), larger spreads can result.

The brine throughput specified in the device information of the heat pump corresponds to a temperature spread of the heat source of approx. 3 K. In addition to the volume flow, the pressure losses in the brine circuit system and the technical data of the pump manufacturer must be taken into account. In doing so, pressure losses in pipelines, internals and heat exchangers connected in series must be added.

Maintenance instructions
In order to be able to guarantee safe operation of the heat pump, it must be serviced at regular intervals. The following work can also be carried out without special training:

• Cleaning the dirt filter in the heat pump's brine circuit

3.1.2 Building drying

When building houses, large amounts of water are usually used for mortar, plaster, plaster and wallpaper, which only slowly evaporates from the structure. In addition, rain can additionally increase the moisture in the building. Due to the high level of humidity in the entire building, the heating requirement of the house is increased in the first two heating seasons.

The building should be dried out with special, on-site devices. If the heating output of the heat pump is limited and the building dries out in autumn or winter, it is advisable to install an additional electric heating element, especially with brine / water heat pumps, to compensate for the increased heat demand. This should only be activated in the first heating period depending on the brine flow temperature (approx. 0 ° C).

3.1.3 Brine liquid

Brine concentration

In order to prevent frost damage to the heat pump's evaporator, an anti-freeze agent must be added to the water on the heat source side. In the case of underground pipe coils, frost protection of -14 ° C to -18 ° C is required due to the temperatures occurring in the cooling circuit. A monoethylene glycol-based antifreeze is used. The brine concentration when laying in the ground is 25 to a maximum of 30% by volume.

A mixture of water and an anti-freeze agent is used as the heat transfer medium in order to achieve a lower freezing point. Ethanediol (ethylene glycol) is used as an antifreeze in the majority of plants in Germany, Austria and Switzerland.

The use of pure monoethylene glycol is therefore recommended if it can be ensured that there is no permanent supply of oxygen during operation due to a closed brine circuit (e.g. AFN 824, AFN 825).

Table 3.1: Approved antifreeze agents recommended by Dimplex

The following antifreeze agents are not approved due to a lack of long-term experience:

• "Thermera", which is made on the basis of betaine and is not without controversy from an environmental point of view.

• "Tyfo special without corrosion protection inhibitors", as this antifreeze attacks non-ferrous metals such as copper.

• "Tyfo special with corrosion protection inhibitors", as this is not officially approved by our suppliers and is so aggressive that it leads to corrosion on the sheet metal cladding in the event of leaks.

Fig. 3.1: Freezing curve of monoethylene glycol / water mixtures as a function of the concentration

Pressure protection
If only heat is extracted from the ground, brine temperatures between approx. 5 ° C and approx. +20 ° C can occur. Due to these temperature fluctuations, there is a change in volume of approx. 0.8 to 1% of the system volume. In order to keep the operating pressure constant, an expansion vessel with a pre-pressure of 0.5 bar and a maximum operating pressure of 3 bar must be used.

Filling the plant
The system should be filled in the following order:

• Mix the required antifreeze-water concentration in an external container

• Check the previously mixed antifreeze / water concentration with an antifreeze tester for ethylene glycol

• Filling the brine circuit (max. 2.5 bar)

• Vent the system (install microbubble separator)

Relative pressure loss
The pressure loss in the brine circuit depends on the temperature and the mixing ratio. As the temperature falls and the proportion of monoethylene glycol increases, the pressure loss in the brine increases.

Fig. 3.2: Relative pressure loss of monoethylene glycol / water mixtures compared to water as a function of the concentration at 0 ° C and -5 ° C

Table 3.2: Total volume and amount of frost protection per 100 m pipe for PE pipes and frost protection down to -14 ° C

3.1.4 Materials in the brine circuit

Material for geothermal collectors
Pipes made of PE 100 / PE-X can be used in stone-free floors. For stony soils, crosslinked pipes made of polyethylene (e.g. PE 100-RC / PE-X) with an outer diameter of 32 mm are recommended due to their higher notch impact strength. PE-RT can be used for applications in which higher temperatures in the brine circuit are to be expected (e.g. energy fences or waste heat utilization). These can be used for operating temperatures of up to 70 ° C.

Other materials
When using other materials such as copper, brass or stainless steel in the brine circuit, the corrosion resistance of the materials must be checked. Corrosion can also occur due to condensation on pipes that are not or inadequately insulated in the brine circuit.

3.1.5 Parallel connection of brine / water heat pumps

When connecting brine / water heat pumps in parallel, care must be taken that there is no incorrect flow in the brine circuit in individual heat pumps. If only one heat pump is in operation, there may be an external flow through the heat exchanger of the second heat pump if there is no check valve in the brine circuit. To prevent this, a non-return valve must be installed in the flow after each brine circuit pump.

Fig. 3.3: Parallel connection of brine / water heat pumps

A similar incorrect flow can also occur when using a passive cooling station (PKS). A non-return valve / non-return valve must also be installed on site after each brine circulating pump.

3.2 Geothermal collector

Geothermal collectors extract seasonally stored energy from the subsurface under the free surface of the earth. In particular, the liquid / solid phase change of the water in the ground is used as a latent heat store in winter. The maximum extraction capacity and the annual extraction work are limited by the storage capacity, the heat transport properties and the thermal regeneration of the subsoil as well as the collector geometry and the operating mode of the system. With regard to the soil, the water content is a major influencing factor.

The coupling to the earth's surface is decisive for the performance of geothermal collectors, as they are affected by heat input from outside air, solar radiation and precipitation in the warmer months
be regenerated. The following design guidelines and application limits therefore apply exclusively to geothermal collectors that are not covered or sealed and that are covered by the natural soil. The heat inflow from the earth's interior is less than 0.1 W / m² and therefore negligible.

The most important criteria for a system decision and the preliminary planning are summarized below:

• In individual cases, geothermal collectors are subject to notification or approval from the lower water authority.

• Building over the geothermal collector is not permitted. The terrain surface above a collector system must not be sealed, as this impairs regeneration.

• A deeply rooted vegetation over a collector is to be avoided. In the worst case, the vegetation delay over a collector is around two weeks.

• The following minimum clearances and standard dimensions are recommended:
  - between collector and buildings: 1.2 m
  - Lines leading between collector and water: 1.5 m
  - between the collector and the property line: 1 m
  - Installation depth of the collector: see section below
  - Installation distance of the collector pipes: see section below

3.2.1 Laying depth

In cold regions, the ground temperatures at a depth of 1 m can reach the freezing point even without using heat. At a depth of 2 m, the minimum temperature is approx. 5 ° C. This temperature rises with increasing depth, but the heat flow from the earth's surface decreases. A thawing of the icing in spring is not guaranteed if it is laid too deeply. Therefore, the laying depth should be approx. 0.2 to 0.3 m below the maximum frost limit. In most regions of Germany this is 1.0 to 1.5 m.

3.2.2 Installation distance

When determining the laying distance d_a It must be taken into account that the ice radii that form around the earth snakes have thawed out after a period of frost to such an extent that rainwater can seep away and no waterlogging occurs. The recommended laying distances are between 0.5 and 0.8 m, depending on the type of soil and climatic region. In regions with sandy soils, a laying distance of 0.3 to 0.4 m may also be necessary.

• The longer the maximum duration of the frost period, the greater the laying distance and the required area.

• In the case of poor heat conduction of the floor (e.g. sand), the installation distance must be reduced for the same installation area and thus the total pipe length increased.

3.2.3 Collector area and pipe length

The area required for a horizontally laid ground collector depends on the following factors:

• Cooling capacity of the heat pump

• Soil type and moisture content of the soil and climatic region

• Maximum length of the frost period

• Annual full hours of use

3.2.4 Relocation of the brine collector and distributor

The brine distributors connect geothermal probes or geothermal collectors easily and safely with a heat pump. A water-glycol mixture is usually used as the heat transfer fluid for transferring geothermal energy. In a closed circuit, the brine flows from the collector or probe pipes via the brine collector to the heat pump and via the brine distributor back to the heat source.

Depending on the number of brine circuits to be flown through, the brine collector or brine distributor must be installed (see Figures 3.4 and 3.5). To completely shut off individual collector or probe circuits (e.g. in the event of leaks), both the collector and the distributor are equipped with ball valves. The PE pipes of the collectors or probes can be mounted directly on the ball valves with the pre-assembled compression fittings.

Fig. 3.4: Assembly of brine distributors up to a maximum of 8 circuits

Fig.3.5: Assembly of the brine distributor for a maximum of 16 (2 x 8) circuits

Various points must be observed when installing the brine distributors:

• Mount the brine distributor firmly on a shaft or building wall (e.g. using a wall bracket).

• The collector or probe pipes must be inserted into the manifold from below in a bend free of tension in order to compensate for linear expansion during summer or winter time (tension cracks).

• Ideally, the arch is made using a welding socket.

• Outside the building, the brine distributors should be installed in accessible shafts - protected from rainwater.

• When installing the shaft, it is recommended to cover or support the collector or probe pipes in the ground with an approx. 20 cm thick layer of sand. If an elbow is welded on to compensate for the linear expansion, it should be above ground level.

Fig. 3.6: Installation of the pipelines on the brine distributor

Fig. 3.7: Installation of the pipelines with welding angles on the brine distributor

• If the brine distributors are installed inside a building, they and all pipelines in the house and through the house wall must be insulated so that they are vapor-diffusion-proof in order to prevent condensation.

• For each collector circuit, the collector pipe should not be longer than 100 m, with probe pipes DN 32 a maximum depth of 80 m should not be exceeded - note pressure loss.

• Hand-tighten all screw connections on the brine collector and distributor. Then tighten with a tightening torque of 60 to a maximum of 70 Nm. Do not damage the union nuts when tightening.

• Coat the union nut between the brine distributor or brine collector and the ball valve (compression fitting) with a grease paste to prevent moisture from penetrating.

3.2.5 Installation of the brine circuit

• The individual brine circuits must be hydraulically balanced with one another. Ideally, collector pipe coils of the same length and material properties are laid (Tichelmann principle). Bar regulating valves (e.g. taco-setter) in the individual brine circuits mean an additional pressure loss and thus higher power consumption by the circulation pump in the heat source circuit.

• Each brine circuit must be provided with at least one shut-off valve.

• The brine circles must all be of the same length in order to ensure an even flow and extraction capacity of the brine circles.

• The geothermal collectors should be installed a few months before the heating season if possible so that the ground can settle.

• The minimum bending radii of the pipes according to the manufacturer's specifications must be observed.

• The filling and venting device must be installed at the highest point on the site.

• When laying the brine lines and the intermediate circuit, it must be ensured that no air pockets form.

• All brine pipes (flow and return) in the house and through the house wall must be insulated so that they are vapor diffusion-proof in order to avoid heat and cold losses and to prevent condensation.

• All pipes carrying brine must be made of corrosion-resistant material.

• Brine distributors and return collectors should be installed outside the house.

• When installing the brine circulating pump of the heat source system, the temperature ranges of the pump in the installation instructions must be observed. The position of the pump head must be set so that no condensate can flow into the connection box. If it is installed in a building, it must be insulated so that it is vapor diffusion-proof in order to prevent condensation and ice formation. In addition, soundproofing measures may be necessary.

• The laying distance between pipes carrying brine and water pipes, canals and buildings should be at least 1.2 - 1.5 m in order to avoid frost damage. If this installation distance cannot be maintained for structural reasons, the pipes must be adequately insulated in this area.

• Geothermal collectors must not be built over and the surface must not be sealed.

• The large ventilator with micro-bubble separator should be located at the highest point of the brine circuit. The brine accessories can be installed both inside and outside the building.

Fig. 3.8: Heat pump circuit on the heat source side

Legend angle Tee Reducing nipple Double nipple Pump connection tap poetry Circulation pump Double nipple Large air vent Double nipple Vessel connection group with quick air vent, safety valve, pressure gauge Expansion tank Lockable cap valve Low pressure pressostat Filter insert |

Fig. 3.9: Structure of the brine circuit feed line including fittings

3.2.6 Standard dimensioning of geothermal collectors

The dimensioning table below is based on the following assumptions:

• PE pipe (brine circles): pipe DIN 8074 32 x 2.9 mm - PE 100 (PN 12.5)

• PE supply pipe between heat pump and brine circuit according to DIN 8074:

• Nominal pressure PN 12.5 (12.5 bar)

• specific extraction capacity of the soil approx. 25 W / m² at a laying distance of 0.8 m

• Brine concentration min. 25% to max. 30% antifreeze (glycol-based)

• Pressure expansion vessel: 0.5 - 0.7 bar pre-pressure

An increase in the number of brine circuits and a shortening of the line lengths are not critical with regard to the pressure losses if all other parameters remain unchanged. If the framework conditions deviate (e.g. specific extraction capacity, brine concentration), a new dimensioning of the permissible total pipe length for the flow and return between the heat pump and brine distributor is required.

The required quantities of antifreeze in Tab.3.2 refer to the specified wall thicknesses. With thinner walls, the amount of water and frost protection must be increased and adjusted so that the minimum brine concentration of 25% by volume is achieved.

* Pump part of "Brine accessory package SZB"

Tab.3.4: Dimensioning table of the brine / water heat pumps for a specific extraction capacity from the ground of 20 W / m² Geothermal collector. (Assumptions: brine concentration 25% by volume of antifreeze, 100 m strand length of the individual brine circuits, pipes made of PE 100 (PN12.5), 32 x 2.9 mm according to DIN 8074 and 8075.

Remarks:

• Collector length 100 m; DN 32 x 2.9

• Volume flow per collector: 0.6 m³/H

• Mixing factor water - glycol: 1.5

• Pressure loss collector: 0.52 mWS (water)

• Pressure loss collector: 0.78 mWS (glycol)

• Extraction power from the ground: 20 W / m2²

3.3 Geothermal probes

The most common type of probe, the double U probe, consists of U-shaped pipe loops bundled in pairs. The single U-probes consisting of only one pipe loop and the coaxial probes consisting of an inner and outer pipe are rarer.

In a geothermal probe system, a heat exchanger system is installed in deep boreholes of mostly 20 m to 100 m in the ground. The plastics PE 100, PE 100-RC and PE-X (PE: polyethylene) are used almost exclusively as pipe material.

The most important criteria for a system decision and the preliminary planning are summarized below:

• Geothermal probes up to a drilling depth of 100 m are subject to approval from the lower water authority, drilling depths over 100 m are subject to approval from the mining authority.

• Building over the probe is only permitted for frost-free operation.

• Required access width for the drilling rig: at least 1.5 m for caterpillars or 2.5 m for trucks

• Required work area for drilling rig, rinsing tub, etc .: at least 6 m × 5 m for caterpillars, at least 8 m × 5 m for trucks

However, the exact dimensioning depends on the geological and hydrogeological conditions, which are usually not known to the installer. The execution should therefore be entrusted to a drilling company certified by the international heat pump association or approved according to DVGW W120. In Germany, VDI-4640 sheets 1 and 2 must be taken into account. Boreholes from a depth of 100 m are subject to the mining law BBergG and must be approved in advance by the competent authority.

Earth temperatures
The earth temperature is 10 ° C all year round from a depth of approx. 15 m.

Fig. 3.10: Representation of the temperature profile at different depths of the earth and depending on a seasonal, mean temperature value at the earth's surface

3.3.1 Design of geothermal probes

Geothermal probes are generally designed by planning offices for geothermal energy. An approximate determination of geothermal probes, even in the small power range, is not permitted. This is necessary because the extraction rate depends on the nature of the soil and the water-bearing layers. These factors can only be clarified on site by an executing company.

The long-term, computational simulation of load profiles enables long-term effects to be recognized and taken into account in the project planning. For example, using the probe in summer for passive cooling has a positive effect on regeneration.

3.3.2 Creation of the probe bore

The distance between the individual probes should be at least 6 m so that there is little mutual influence and regeneration in summer is ensured. If several probes are required, they should not be arranged parallel, but transversely to the direction of groundwater flow.

The following additional minimum distances are recommended:

• between probe and buildings: 2 m (the statics must not be impaired).

• between the probe and the pipes carrying water: 2 m to 3 m (differently regulated locally)

• between connecting pipes and pipes carrying water: 1.5 m

• Distances to the neighboring property vary from country to country (recommendation VDI 4640 Part 2, distance between geothermal probes 6 m, distance to the neighbor's probe 10 m, exceptions are possible in coordination with the neighbors).

Fig. 3.11: Arrangement and minimum distance of probes depending on the direction of groundwater flow

Fig. 3.12 shows a cross-section through a double U-probe, as it is usually used for heat pumps. With this type of probe, a hole with a radius r₁ created. Four probe pipes and a backfill pipe are inserted into this and the borehole is backfilled with a cement-bentonite mixture. The probe fluid flows down in two probe tubes and up again in the other two. The tubes are connected to a probe base at the lower end, so that a closed probe circuit is created.

Fig. 3.12: Probe cross-section of a double U probe with a filling pipe

3.3.3 Filling geothermal probes

As with ground collectors, ground probes are generally filled with a 25 to 30 vol% glycol solution. This means that brine inlet temperatures of -5 ° C can easily be achieved in the heat pump. The heat pump is protected from freezing due to the glycol content.
In some cases, however, it may also be necessary to operate the geothermal probe with pure water without frost protection. In this case, the brine inlet temperature must not fall below 0 ° C, as otherwise the water in the brine line can freeze and damage it. For this reason, various points must be observed when operating geothermal probes with water:

• Instead of a brine / water heat pump, a water / water heat pump is used

• In this case, the minimum brine outlet temperature must not be less than 4 ° C

• The transmission performance of the probe is reduced due to the higher temperatures. The number of probes required roughly doubles compared to a soil probe with water-glycol.

• The pre-pressure of the brine expansion vessel must be reduced from 2.5 bar to 0.5 - 0.7 bar.

3.4 Accessories for the ground heat source

3.4.1 Installation instructions for connecting the heat source circuit

Temperatures of below - 15 ° C are sometimes present on the brine pipes when the heat pump is in operation. For this reason, both brine pipes inside the building must be insulated so that they are diffusion-proof, as otherwise condensation would occur.

The wall penetrations into the building should be insulated with well foam or cold-resistant pipe penetrations. All pipe penetrations through walls and ceilings are to be designed with structure-borne noise insulation.

The vibrations caused by the compressor during operation of the heat pump (oscillating movement) are largely compensated for by the internal vibration decoupling. In the case of unfavorable installation conditions, residual vibrations may still occur, which can then be transmitted as structure-borne noise via the pipelines. In this case, wall clamps for fastening the brine piping should not be positioned too close to the heat pump during installation in order to avoid a too rigid connection. Cold pipe clamps also prevent structural damage from condensation. In particularly difficult cases, the installation of expansion joints can help, which are installed as close as possible to the heat pump.

3.4.2 Brine packages and accessories

The following brine accessory packages including a circulation pump are available for using the brine heat source.

Tab.3.5: Brine accessory packages for various heat pumps

* Included in the scope of delivery of the heat pump

3.4.3 Pump assignments for 2-compressor brine / water heat pumps

Tab.3.6: Overview table of the 2-compressor brine / water heat pumps with generator circuit and brine circulating pumps for B7 / W35 for standard systems (included in the delivery of the heat pump)

3.4.4 Brine accessory packages for 2-compressor brine / water heat pumps PP 65-80F

Tab.3.7: Overview table of the brine accessory packages for 2-compressor brine / water heat pumps

Brine deficiency and leakage
In order to detect a possible lack of fluid or a leak in the brine circuit or to meet official requirements, the "low-pressure pressostat brine", available as a special accessory, can be installed in the brine circuit. Heat pump locks.

1. Pipe section with internal and external thread

2. Pressostat with plug and plug seal

Fig.3.13: Low pressure pressostat brine (structure and interconnection)

The pre-pressure of the brine expansion vessel must be reduced from 2.5 bar to 0.5 - 0.7 bar.

The pipe section shown in the sketch is to be installed between the cap valve and the expansion vessel in the brine circuit. The pressure switch is to be connected to the connecting piece on the pipe section. Thanks to the lockable cap valve, the low-pressure pressostat can be easily installed or removed and its function checked. When checking the function of the low-pressure pressostat, keep the drain cock open until the pressostat blocks the heat pump manager and thus the heat pump via a digital signal due to the pressure drop in the brine circuit. Catch the brine in a suitable container. If the low-pressure pressostat does not block the heat pump when there is a visible drop in pressure, the sensor must be checked for function and, if necessary, replaced. After completing the check, fill the brine circuit again with the collected brine liquid. Then check the brine circuit for leaks and the heat pump for its function.

3.5 Other heat source systems for geothermal energy use

As an alternative to geothermal collectors, other types of heat source systems such as geothermal baskets, trench collectors, energy piles, spiral collectors, etc. are also offered. These heat source systems must be designed in accordance with the manufacturer's or supplier's specifications. The manufacturer must guarantee the long-term functioning of the system in accordance with the following information:

• Minimum permissible brine temperature

• Cooling capacity and brine throughput of the heat pump used

• Operating hours of the heat pumps per year

In addition, the following information must be provided:

• Pressure loss at the specified brine throughput for the design of the brine circulating pump

• Possible influences on the vegetation

• Installation regulations

Possible optimization of the extraction performance depends primarily on the climatic conditions and the type of soil and not on the type of heat source system.

3.6 Heat source water with intermediate heat exchanger

3.6.1 Development of water as a heat source in the event of contamination

For indirect use of the water heat source, brine / water heat pumps can be operated via an intermediate circuit with an additional stainless steel heat exchanger. For this purpose, an additional heat exchanger is installed in the heat source circuit of the heat pump and the intermediate circuit is filled with monoethylene glycol.

The external stainless steel heat exchanger makes it possible to use the groundwater heat source even in areas with heavier water pollution. In areas with a year-round water temperature below 13 ° C, no water analysis for corrosion is necessary.

There are various package solutions available, consisting of a heat pump, heat exchanger, suitable brine accessories and a safety thermostat to protect the heat pump from freezing. In this case, the heating output of the heat pumps is specified differently at operating point B7 / W35. This corresponds to a brine inlet temperature of 7 ° C with an assumed water temperature of 10 ° C and a gradient or spread over the heat exchanger of 3 K.

Tab.3.8: Heat pump packages with intermediate heat exchanger

Fig.3.14: Heat pump with intermediate heat exchanger

The flow switch in the primary circuit (FS) prevents the heat pump from being switched on if there is no volume flow from the cooling or groundwater pump.

In the case of brine / water heat pumps, the intermediate heat exchanger circuit must be filled with antifreeze (at least -14 ° C).

The brine circuit is to be designed in the same way as with conventional ground collectors or geothermal probes with a circulation pump and safety fittings. The circulation pump must be dimensioned so that it does not freeze in the intermediate heat exchanger.

When using a brine / water heat pump, temperatures below 0 ° C can occur in the secondary circuit. To protect the intermediate heat exchanger, it must be protected by an additional frost protection thermostat (T). This must be installed at the water outlet of the primary circuit in order to reliably prevent the heat exchanger from freezing. When the thermostat is switched off, the heat pump is blocked via the digital input ID3 of the heat pump manager. The thermostat should also be passed on as a fault message to any existing building management system in order to prevent the heat pump from cycling. The switch-off point of the thermostat (e.g. 4 ° C) depends on the on-site system configuration, the measurement tolerances and hysteresis.

The maximum permissible flow temperatures on the heat source side of a brine / water heat pump are 25 ° C. To prevent the heat pump from switching off due to excessive brine inlet temperatures, there are various options that are described in the following chapter.

3.6.2 Extension of the temperature range

If the temperature of the heat source fluctuates, the use of a brine / water heat pump is recommended, as minimum brine outlet temperatures of -9 ° C are possible here. In comparison, water / water heat pumps switch off at a minimum water outlet temperature of 4 ° C. The maximum brine inlet temperature for both brine / water and water / water heat pumps is 25 ° C. Exceeding or falling below the application limits can be prevented in various ways.

Fig.3.15: Heat pump with thermostatically controlled 3-way valve in the brine circuit M21 (to be provided by the customer)

Variant 1 - heat pump with 3-way valve
A thermostatically controlled 3-way valve is installed in the brine circuit. If the brine inlet temperature rises above 25 ° C, a partial volume flow of the brine return flow is added to the brine flow via the mixer. The mixer is controlled by an external control.

Variant 2 - heat pump with buffer storage in the brine circuit
Variant 2 provides for the use of a buffer storage tank in the brine circuit (see Fig. 3.16 on p. 22). The buffer storage tank is loaded by means of an external control system via pump P1. From a minimum temperature of 3 ° C in the buffer tank, the pump is activated and loads it. Pump P1 switches off when the temperature reaches a maximum of 24 ° C. The heat source pump (primary circulation pump M11) in the brine circuit is controlled by the heat pump manager. If a temperature of 3 ° C below or a temperature of 25 ° C is reached on the temperature sensor (R6), the heat pump manager switches off the heat source pump. The brine circuit must be filled with glycol with at least 25% by volume.

Fig.3.16: Heat pump with buffer storage in the brine circuit

3.7 Heat source absorber systems (indirect use of air or solar energy)

Availability
Restrictions due to weather influences and limited areas possible.

Possibility of use

• bivalent

• monovalent in combination with an additional geothermal collector

Development effort

• Absorber system (energy roof, pipe register, massive absorber, energy fence, energy tower, energy stack, etc.)

• Brine based on ethylene glycol or propylene glycol in frost-proof concentration

• Piping system and circulation pump

• Construction work

Pay particular attention to:

• structural requirements

• Weather influences

Dimensioning of absorber systems
When it comes to the dimensioning of roof absorbers, energy columns or fences, the individual constructions differ considerably, so that basically the information guaranteed by the manufacturer must be used for the design.
As practice shows, however, the following data can be used as a basis:

• The design of the absorber surface should in principle be based on the specified night performance of the absorber.

• At air temperatures above 0 ° C, rain, condensation or snow can freeze on the absorber surface at low brine temperatures, which negatively affects the flow of heat.

• Monovalent operation is only possible in combination with the use of geothermal energy.

• With solar energy gains in the transition period, brine temperatures of 50 ° C and more occur, which exceed the application range of the heat pump.

Brine concentration
In the case of roof absorbers, energy fences, etc., a frost protection of -25 ° C is required due to the low outside temperatures. The brine concentration in this system is 40%. With increasing brine concentration, increased pressure losses must be taken into account when designing the brine circulating pump.

Filling the system:
The system is filled as described in the “Brine fluid” chapter.

Design of the expansion vessel:
When the absorber is operated exclusively, the brine temperatures fluctuate between approx. –15 ° C and approx. +50 ° C. Due to these temperature fluctuations, an expansion vessel is required in the heat source system. The form is to be adapted to the height of the system. The maximum overpressure is 2.5 bar.

Air-loaded absorber

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