4 water / water heat pumps

4.1 Groundwater as a heat source

If the appropriate boundary conditions exist, the thermal energy generation via the groundwater can represent a very efficient form of thermal use of the subsoil for heating and / or cooling purposes. When planning, building and operating well systems for thermal use of the subsoil, the water law requirements and the respective country-specific regulations must be observed. For thermal use, with a view to the efficiency of the system as well as groundwater protection, groundwater near the surface with a free groundwater level (same groundwater-bearing layer) should primarily be used.

When using deeper groundwater levels, special protective measures are required. The planning and execution of well systems must be carried out by relevant planning offices or appropriately qualified specialist companies in the well construction trade. The recommendations of guideline VDI 4640 Part 2 must be observed here. Information about the productivity of the groundwater stock and the chemical composition of the groundwater is absolutely essential. A test borehole, which can later be converted into a well, is recommended for assessment. The manufacturer's recommendations for the quality of the well water must be observed (see Section 4.2). If the water quality does not meet the manufacturer's specifications, either a heat pump model with a suitable evaporator (stainless steel heat exchanger) or an intermediate circuit with a screwed stainless steel plate heat exchanger (see Section 3.6 and Section 4.3.4) can be used. The evaporator must be protected from frost damage, for example by means of temperature monitoring or an intermediate circuit that is operated with an anti-freeze mixture. The manufacturer's specifications are to be observed. The intermediate circuit requires additional energy to operate the brine pump and reduces the heat source temperature by around 3 K, which leads to a reduced coefficient of performance.

For small systems, groundwater is a rather difficult heat source to assess if there is no experience with systems in the immediate vicinity, because the effort for a test borehole is very high. The test hole cannot be used if it is unsuitable. For large systems, the costs for a test borehole and for a pumping test are of lesser importance; greater depths (up to 50 m) are economically justifiable here. The most important criteria for a system decision and preliminary planning are summarized below:

• Approval according to the Water Management Act (WHG) by the lower water authority

• Distance between suction and suction wells: at least 15 m in the direction of flow of the groundwater flow

• Recommended minimum distance between boreholes and existing buildings: 2 m

• The stability of buildings must not be endangered.

Availability

• All year round

Possibility of use

• monovalent

• monoenergetic

• bivalent (alternative, parallel, partially parallel)

• bivalent regenerative

Development effort

• Approval procedure (lower water authority)

• Delivery wells / absorption wells with airtight closure of the well heads

• Water quality (water analysis)

• Piping system

• Well pump

• Earthworks / construction work

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 inside of the heat pump

• Cleaning the primary circuit (dirt trap, particulate filter, ...)

In addition, the tightness of the heat pump and the functionality of the refrigerant circuit must be checked at regular intervals.

Work on refrigerant-carrying components may only be carried out by appropriately trained and instructed personnel.

4.1.1 Dimensioning information - heat source water

The heat source of the water / 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 provide. 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 output of the heat source must therefore be checked and, if necessary, adjusted to the new cooling output.
Dimensioning the well pump

The water volume flow depends on the performance of the heat pump and is conveyed by the well pump. Depending on the output, the mass flow should be selected so that at the lowest heat source temperature (7 ° C) there is a temperature spread across the evaporator of 2 - 3 Kelvin. The water 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 well 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.

** The back pressure corresponds to the free compression of the well pump at the operating point minus the pressure difference between the highest point of the well system and the water level (level) in the well (manometer).
Temperature difference heat source groundwater

The Water Management Act (WHG) defines the difference between the inlet and outlet temperature of a water / water heat pump. These values are defined as follows:

• Permissible change in temperature of the water to be introduced compared to the extraction temperature of the groundwater: +/- 6 K

• Minimum temperature of the water to be introduced: 5 ° C

• Maximum temperature of the water to be introduced: 20 ° C

4.1.2 Development of the heat source groundwater

From a well depth of 8 to 10 m, the groundwater heat source is suitable for monovalent heat pump operation, as this only shows slight temperature fluctuations (7-12 ° C) all year round. In principle, the approval of the responsible water authority must be available for heat extraction from groundwater. It is generally issued outside of water protection zones, but is tied to certain conditions, such as a maximum withdrawal quantity or a water analysis. The amount withdrawn depends on the heating output. For the operating point W10 / W35, Table 4.1 contains the required withdrawal quantities. The planning and construction of the well system with delivery and absorption wells should be entrusted to a drilling company certified by the international heat pump association with a seal of approval or approved according to DVGW W120. In Germany, VDI 4640 sheets 1 and 2 must be taken into account.

¹ Stainless steel spiral heat exchanger as standard! ² Pay attention to the counter pressure of the well system in order to avoid bearing damage to the well pump!
Tab.4.1: Dimensioning table of the minimum required well pumps for water / water heat pumps for W10 / W35 for standard systems with closed wells. The final definition of the well pump must be made in consultation with the well builder.

4.2 Water quality requirements

Regardless of the legal regulations, the groundwater must not contain any settable substances and the iron (<0.20 mg / l) and manganese (<0.10 mg / l) limit values must be observed in order to prevent the heat source system from becoming clogged . Experience shows that soiling with grain sizes over 1 mm, especially with organic components, can easily lead to damage. Grainy material (fine sand) does not settle if the specified water flow rates are adhered to. The dirt trap included in the scope of delivery of the heat pump (mesh size 0.6 mm) protects the evaporator of the heat pump and must be installed directly at the inlet of the heat pump.

The use of surface water or salty waters is not permitted. Initial information about a possible use of the groundwater can be obtained from the local water supply company.

1. Water / water heat pumps with welded stainless steel spiral heat exchanger (Tab.4.1)
   A water analysis regarding corrosion of the evaporator is not necessary if the annual mean temperature of the groundwater is below 13 ° C. In this case, only the limit values for iron and manganese have to be complied with (ocher formation). At temperatures above 13 ° C (e.g. waste heat recovery), a water analysis must be carried out in accordance with Tab.4.2 and the resistance of the stainless steel evaporator of the heat pump must be verified. If one characteristic is negative “-” or two characteristics are “0” in the “Stainless steel” column, the analysis is to be assessed as “Negative”.

2. Water / water heat pumps with copper-brazed stainless steel plate heat exchangers Irrespective of the legal provisions, a water analysis according to Tab. 4.2 must be carried out in order to prove the resistance for the copper-soldered evaporator of the heat pump. If one characteristic is negative “-” or two characteristics are “0” in the “Copper” column, the analysis is to be assessed as “Negative”.

Tab 4.2: Resistance of copper-brazed or welded stainless steel plate heat exchangers to water constituents "+" normally good resistance; "0" Corrosion problems can arise, especially if several factors are rated with 0; "-" should not be used; [<less than,> greater than]

Even if the limit values for the water quality specified in Table 4.2 are adhered to, constant deposits of iron, manganese and lime can impair the performance of the heat pump, up to and including complete failure of the well and heat pump system. Therefore, the well system must be checked regularly and, if necessary, the well pump system cleaned.

4.3 Development of the heat source

4.3.1 Direct use of water of consistently good quality

Water with temperatures between 7 ° C and 25 ° C can be used directly with a water / water heat pump if the compatibility of the groundwater, cooling water or wastewater has been proven according to Tab. 4.2. In the event of a negative assessment of the water quality or if the water quality changes, a heat pump with an intermediate circuit (see Section 4.3.2 ff) must be used.

4.3.1.1 Heat source groundwater

Extraction wells The groundwater, which the heat pump uses as a heat source, is taken from the ground via a well. The well output must ensure continuous extraction for the minimum water flow through the heat pump.
Inlet fountain The groundwater cooled by the heat pump is fed back into the ground via an injection well. This must be drilled at least 15 m behind the delivery well in the direction of the groundwater flow in order to exclude a "flow short circuit". The intake well must be able to absorb the same amount of water as the delivery well can deliver.

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Fig. 4.1: Example of an integration of the water / water heat pump with delivery and absorption wells

4.3.1.2 Heat source waste heat from cooling water

Temperature range
When using water with temperatures between 8 ... 25 ° C, it must first be clarified whether the cooling water is available in sufficient quality and quantity and to what extent the heat generated by the heat pump can be used. If the compatibility of the cooling or waste water according to Table 4.2 is permanently ensured, a water / water heat pump can be used.

4.3.2 Indirect use of water as a heat source

If the compatibility of the water cannot be proven or there is a risk that the quality of the water can change, an intermediate heat exchanger must be connected upstream to protect the heat pump. The intermediate circuit increases operational reliability, especially when a brine / water heat pump is used and the secondary circuit is thus filled with brine. A water / water heat pump with an intermediate heat exchanger should only be used if the use of brine as a heat transfer medium is not permitted and permanent water temperatures above 10 ° C (e.g. waste heat from production processes) can be guaranteed.

4.3.3 Planning recommendation groundwater / intermediate circuit heat exchanger

Brine heat pump with intermediate circuit heat exchanger (WSI packages)
(Use of groundwater, closed system)
The minimum brine outlet temperature must be set to> 1 ° C. A thermostat must be provided in the heat source circuit that switches off the heat pump in the event of a fault (strap-on thermostat included in the scope of delivery of the WSI packages).
Water heat pump with intermediate circuit heat exchanger
(Groundwater use, open system)
Installation of a flow switch is recommended because there can be time delays until sufficient groundwater is pumped or the volume flow can drop abruptly during operation.
Water heat pump with stainless steel spiral heat exchanger for groundwater
(Groundwater use, open system)
A stainless steel spiral heat exchanger ("Spirec") increases the reliability of the heat pump system. The use of a flow switch (DFS) contributes to a further increase in operational safety.

4.3.4 Heat exchanger (System separator) to protect the heat pump

The external The heat exchanger must be planned according to the heat pump used, the existing temperature level and the water quality. In the simplest case, the heat exchanger consists of PE pipes that are laid directly in the heat source and therefore do not require an additional heat source pump. This cost-effective alternative can be used if the volume of the heat source is sufficiently large (e.g. waste water basin, flowing water).
Otherwise, screwed plate heat exchangers are to be used.
The heat exchanger is configured depending on the following parameters:

• Water quality

• Operating temperature range

• Cooling capacity of the type of heat pump used

• Water flow primary and secondary circuit

Depending on the software version of the heat pump control, the frost protection of a brine / water heat pump can be adjusted. If the standard value is increased from -8 ° C to +4 ° C, the heat pump is switched off at brine outlet temperatures below +4 ° C.
Assembly of the heat exchanger For optimal heat transfer, the heat exchangers must be connected using the counterflow principle. They must also be protected from contamination. For this purpose, a dirt trap with a mesh size of approx. 0.6 mm must be installed in front of the inlet of the heat exchanger. Compensators should be used to reduce the transmission of structure-borne noise and vibrations (e.g. heat source pumps ...).
Maintenance of the heat exchangers Depending on the degree of pollution in the water, the heat exchanger can become dirty, reducing its transmission capacity. Regular cleaning should take place to prevent this. For example, the so-called CIP process (cleaning-in-place) is used. The heat exchanger is rinsed on site with a weak acid such as formic, citric or acetic acid to remove deposits.

* Suggestion for a well pump ** Control with 0 - 10V input signal absolutely necessary
Tab. 4.3: Overview table of the 2-compressor water / water heat pumps with generator circuit pumps (included in the scope of delivery of the heat pump) and the minimum required well pumps for W10 / W35 for standard systems with closed wells. The final definition of the well pump must be made in consultation with the well builder.

4.3.4.1 Stainless steel plate heat exchangers WTE 20 to WTE 40

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Fig.4.2: WTE 20 - WTE 37

Fig.4.3: WTE 40

Device information stainless steel plate heat exchanger

Tab.4.4: Technical data for screwed stainless steel plate heat exchangers WTE 20 - WTE 40

4.3.4.2 Stainless steel plate heat exchangers WTE 50 to WTE 130

Device information stainless steel plate heat exchanger

Tab.4.5: Technical data of screwed stainless steel plate heat exchangers WTE 50 - WTE 130

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