Open loop

Open loop sea water collectors take the form of two separate straight pipe collectors that feed heat from the water to the ground source heat pump system.

What is an open loop collector?

As per all open loop system collectors, no anti-freeze fluid is featured in open loop non-tidal collector installations. These installations involve the water from the water source being abstracted and filtered through the ground source heat pump. This water is then discharged either back to the water source or another acceptable discharge area.

Consideration needs to be taken with regards to corrosion issues, filtration, extraction and possible freezing within the heat exchanger. It is also important to consider the electrical energy required to pump the water to the ground source heat pump.

Considerations

If you are considering an open loop borehole for your ground source heat pump system, there are a number of points which need to be taken into consideration:

Seek relevant permissions from the Environmental Agency:

  • In order to drill or test pump a water supply borehole, you will require a consent to investigate a groundwater source under section 32 of the Water Resources Act;
  • To operate an open loop scheme you will need an abstraction licence (if the abstraction is greater than 20m3/d) and an environmental permit to discharge water.

In addition to permission requirements you should consider the design of any scheme to ensure its longevity and efficient performance:

  • Employ the services of a professional hydrogeologist and/or groundwater engineer to ensure you have a well-designed open loop ground source heat pump system;
  • Ensure that there is adequate separation of the abstraction and discharge boreholes to ensure that large quantities of discharged water do not feed back into the abstraction borehole affecting the temperature of the abstracted water and hence performance of the system;
    • Ensure the discharge borehole is well designed so that it does not clog within a short period of operation. Careful control of the gas content, water chemistry and the particulate content of the water is required as small bubbles of gas and particulates can result in rapid clogging of the borehole or aquifer. Biofouling with bacterial growth can also become problematic;
    • The hydraulic efficiency of the scheme will reduce over time, even with a good design, due to the clogging of the face of discharge as detailed above and especially if thermal breakthrough occurs between the abstraction and discharge boreholes. Background temperature of the groundwater may also change over time if more schemes are constructed in the area.
  • To avoid possible freezing and corrosion of the heat pump it is advised that the heat pump and water source are separated by an intermediate plate heat exchanger as shown below. It is also important that the water is filtered and bacterial growth minimised to avoid fouling. This does mean that a regular maintenance schedule is required.
  •  The signal for them extraction pump can be taken (via a relay) from the 24V supply between the controller and compressor contactor, contact Kensa Engineering for further details.
  • A check valve is also required on the water source side to ensure that the system does not drain down when the extraction pump is turned off.
  • As an alternative to open loop it is possible to place the ground loops directly in the water source mounted on mats, called pond mats. This removes the need for an intermediate heat exchanger, removes the filtration issues and allows the use of antifreeze within the system to avoid problems with freezing.

With pond mats there are issues which need to addressed and these include:

  1. Protecting the pond mats from damage (waves, debris, boats, propellers etc)
  2. Physically securing the pond mats against tidal current and waves
  3. Corrosion of the frame, mountings and structure
  4. Marine fouling

A good place to install pond mats is mounting them to the underside of a floating pontoon.

Facts at a glance:

Installation

Open loop systems can be cheaper to install than digging trenches if an readily available source of water is used.

Efficiency

Open loop systems can have a higher efficiency than ground source due to the higher water source temperature.

Licences

When extracting or drilling boreholes for extraction the Environmental Agency needs to be consulted for the appropriate licences.

Freezing

To avoid freezing the water around the heat pump heat exchanger an intermediate heat exchanger should be used to separate the circuits.

Filtration

With open source, there is a requirement for filtration and additional maintenance issues which must be addressed.

Pond mats

To reduce maintenance it is possible to use a pond mat system, where pipe is connected to mats and submerged to the bottom of an open water source such as a lake.

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