Water-to-water heat pump. Heat take-off from a pond

  In units of the given type the low-grade heat energy is taken off from ponds. The heat take-off is actually performed by laying down a manifold being a system of plastic pipelines. Laying-down is carried out at a river or lake bottom. The minimal laying depth is from 2 meters. To prevent the floating-up, weights are fixed to the pipelines system at the rate of 5 kg per 1 meter length, thus, manifolds are forced down to the pond bottom. The specific heat take-off from such manifold is on an average 30 W from 1 m length of a pipeline.

   From what has been said it follows that a circuit of proximately 220 m total length is needed for a heat pump with 10 kW heating capacity. The heating capacity (a.k.a. the condensing capacity Qc) is equal to the sum of load on the evaporator Qe and power N consumed by the compressor from electric mains.

Qc=Qe+N, kW

  As an example let us consider the heat pump operation based on Bitzer 4FES-5Y-40S compressor of Ecoline series.

  The unit technical characteristics at the first operation mode (it is presented in Table 1.1 of the article about technical characteristics upon the change of operation conditions) indicate that a manifold to be laid in a pond should provide the load of 6.86 kW to ensure the evaporator Qe performance. The other 3.56 kW (a part of the heat pump heating capacity) stand for the power N consumed by the compressor from electric mains.

  Let us view a heating system via a closed-type heat pump with the low-grade heat take-off in more detail. A heat pump provides a heat supply system of resort cottage within the territory of a nature reserve located nearby a natural pond (lake).

Установка коллектора теплового насоса на дно озера для отбора низкопотенциального теплаIndications in the figure: ① – cottage, ② – lake, ③ – manifold

Figure 5.1 The general view.

  The unit operation description. Depending on use conditions, coefficient COP may have various values. For the given operation mode of the unit it is 3.1. For heating of compartments with total area of 450 m2 and for utility the heat pump should ensure the heating capacity of 48 kW.

  The calculated mean temperature of a lake bottom, where the manifold is installed, is 5°C. The refrigerant boiling temperature will be 1 °C. The design total length of pipelines composing the manifold to be laid onto a lake bottom is 1170 m.

  Owing to the below diagram there can be seen a vast scope of application of the heat pump within the framework of the given object (hot water for kitchen, bathroom, shower, warm floor, water heating in swimming pools, and also various heating devices such as towel dryers).

Схема принципиальная работы теплового насоса вода-вода с отбором тепла из водоема (озера)

Figure 5.2 The water-to-water heat pump with heat take-off from a pond. Schematic diagram.

Designations on the diagram

① – semi-hermetic reciprocating compressor

② – water-cooled shell and tube condenser (water heater)

③ – thermal expansion valve

④ – plate-type water evaporator

⑤ – centrifugal pump for cooling medium

⑥ – centrifugal pump for heat-transfer fluid (hot water)

⑦ – manifold for heat take-off from a pond

⑧ – storage tank for heat-transfer fluid

⑨ – house heating system batteries

⑩ – plate-type direct-flow water heater

⑪ – kitchen

⑫ – bathroom

⑬ – shower

⑭ – warm floor

⑮ – swimming pool

⑯ – towel dryers

Table 5.1. The designations of working media.

Medium reference designation Name State
Suction of cooling medium from manifold Liquid
Supply of cooling medium into evaporator by means of centrifugal pump
Return of cooling medium given off the heat into manifold
Supply of warmed heat-transfer fluid (water) into heat supply system of the object
Outlet of heat-transfer fluid from consumers of heat supply system and intake to pump suction
Supply of heat-transfer fluid into condenser by means of centrifugal pump
Refrigerant suction by means of compressor Gas
Discharge of compressed hot refrigerant into condenser Gas
Supply of condensed refrigerant to thermal expansion valve Liquid
Refrigerant supply into evaporator Vapor-liquid mixture
Outlet of heat-transfer fluid from storage tank and its supply to consumers Liquid
Water inlet from water supply system into direct-flow water heater
Water outlet from water heater and supply to consumers for domestic needs
Wasted water drain into sewerage

   The operation principle description. The centrifugal pump (pos. 5) pumps out a cooling medium from the manifold (pos.7), located in a pond (lake) and delivers it into the coolant circuit of the plate-type evaporator (pos. 4). The refrigerant boils in the evaporator taking off the desired low-grade heat from a natural source – in this case from a pond. Having been cooled down in the evaporator (therefore, given off the heat to the refrigerant), the cooling medium returns to the pond manifold. Then the evaporated refrigerant passes to the reciprocating compressor (pos. 1). The compressor compresses refrigerant vapours, hence, heating them to the temperature in a range of 90-130 °С and discharges them into the water-cooled condenser (pos. 2). In the condenser water circuit a heat-transfer fluid is heated, cooling and condensing the refrigerant to the liquid state in a freon cavity of the given heat exchanger. The heat-transfer fluid (both fresh water and ethylene glycol and propylene glycol solutions may be used as a heat-transfer fluid) comes to the condenser water circuit with the help of the pump (pos. 6). Then, having being heated, it comes into the storage tank (pos. 8) and the plate-type direct-flow water heater (pos. 10) at the same time. The heat-transfer fluid, being spreading, passes from the storage tank to the following consumers of a cottage water supply system: batteries (pos. 9) (or fancoils), floor warming system (pos. 14), the heating system of swimming pool (pos.15), various domestic heating devices, towel dryers in particular (pos. 16). The water heater is designed to heat the tap flowing water for needs of such consumers as kitchen (pos. 11), bathroom (pos. 12), shower (pos. 13). Having given off its heat energy to all consumers, the heat-transfer fluid returns to the pump suction. Having given off its heat energy to the tap flowing water in the direct-flow water heater, the heat-transfer fluid comes back and the waste water, after using for domestic purposes, is disposed of into sewerage.

  Also, up to date, the diagrams are applied, in which the heat energy from sewage waters may be directly used to receive hot water via the heat pump.

  The liquid refrigerant is directed to the thermal expansion valve (pos. 3), in which the expansion pressure sharp drop takes place. After TEV the refrigerant comes to the evaporator and the cycle begins to repeat.

  The unit main equipment: Bitzer 6JE-33Y-40P semi-hermetic reciprocating compressor of Ecoline series, Bitzer K1353T shell-and-tube condenser, Danfoss EnFusion™ B3-052-90 plate-type evaporator. The technical characteristics of this equipment are given in the article Water-to-water heat pump. Heat take-off from a pond. Technical characteristics of equipment.

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