An Option for Energy Efficiency and Water Production at Cape Town

Summary:

The thermal capacity of seawater off Cape Town’s coast can be applied to reduce the energy consumption of commercial scale refrigerators and air-conditioners as well as sustain the energy efficient operation of refrigeration-based technology that extracts potable water from humid air.

Introduction:

During successive summer seasons that extend over a period of decades, the City of Cape Town has encouraged citizens to save water. The usual weather patter involves winter rainfall that is stored in mountain dams to provide Cape Town with water over the hot and dry summer season. Perhaps as a result of changing weather patterns occurring internationally, the winter of 2016 recorded unusually low levels of rainfall in the catchment areas for Cape Town’s main dams. The result was that by early March 2017, some of Cape Town’s dams were recording water volumes of around 30%.

The Cape Town area is not alone in experiencing a season of unusually low rainfall. Over a period of 6-successive years, the State of California in the southwestern USA experienced an extended period of reduced rainfall that emptied several dams and resulted in several river beds having run dry. Yet the American southwest survived as a result of several initiatives from an earlier era. These initiatives include a wastewater recycling program that involves reverse-osmosis technology and intense ultra-violet radiation treatment of recycled water to remove bacteria. Water diversion for other areas also enabled the region to endure the drought.

Cape Town Humidity:

Changing weather patterns have resulted in Cape Town experiencing more powerful seasonal winds and increased summertime humidity that winds blow in from the sea. Wind that blows over the southern region of Cape Town carries much of the humidity back out to sea. When the night time temperature drops to below the dew point, some of the humidity condenses on mountain vegetation, on rocks, on roofs, on cars and drips to the ground and forms into mountain streams that flow to the sea or into storm sewers across the Greater Cape Town area.

There is much technology capable of extracting potable water from humid air and the list includes fog or dew fences as well as electrically powered dehumidifier technology that operates like and air conditioner to extract water from humid air. Various variations of fog fence technology are applied internationally in regions that receive insufficient rainfall. Research into atmospheric humidity has suggested that the atmosphere may hold in excess of 1.8-million litres of water per capita. During Cape Town’s rainy winter season, comparatively small percentage of that humidity falls as rain over the catchment areas of Cape Town’s dams.

Cape Town’s Untapped Resource:

Several locations around the Greater Cape Town area have access to a resource that can simultaneously improve energy efficiency and reduce energy consumption related to large-scale commercial air conditioning, heat pump and refrigeration technology. It could be expanded to sustain the operation of water-from-air extraction technology. It is based on undersea pipeline technology that carries natural gas and oil. Such undersea technology is expected to be installed off the coasts of Mozambique and Tanzania where large underground reservoirs of natural gas have been discovered. Except that the undersea pipeline technology has an alternative application.

A few kilometres of sealed or closed-loop undersea pipeline with inlet and outlet at the same location can also function like a heat exchanger. Hot water pumped in at the inlet could translate to cool water at the outlet. An extended-length closed-loop pipe that rests on the seafloor near the coast can allow for water in the pipeline to flow to high elevation above sea level, thereby reducing the cost of pumping seawater from sea-level to high elevation. It can also maintain the relative purity of the water flowing inside the closed-loop pipeline.

Closed-Loop Pipeline:

A closed-loop pipeline that functions as an undersea heat exchanger can connect to a large-scale coastal heat exchanger that could connect to multiple buildings via a distribution and return network of high-pressure and possibly insulated water pipeline. The system would involve 2-separate closed-loop pipelines to enhance system reliability, with the ground-based pipeline system connecting to large-scale refrigeration, air-conditioning as well as heat pump and water-from-air extraction systems. Non-saline water has a density of almost 850-times that of air at atmospheric pressure and a specific heat of 4-orders of magnitude, for a per unit volume heat capacity 3,400-times that of air.

During warm summer weather, an air-cooled air-conditioning or refrigeration system will consume 3 to 4-times the amount of electrical energy as a water-cooled air-conditioning or refrigeration system. Cape Town has several locations where it is possible to lay down a closed-loop pipeline on the seafloor and near the coast. The seawater temperature in False Bay is 23-deg C to 25-deg C throughout the year and on the Atlantic side of the Cape Peninsula to Saldanha Bay at 15-deg C throughout the year. Coastal seawater can serve both and simultaneously as a heat sink and thermal reservoir.

Seawater Thermal Reservoir:

During winter across South Africa, many homes and commercial buildings use electrical energy for interior heating. New generation large-scale air-conditioners can also operate as a heat pump, that is, provide interior cooling for building during summer and as a heat pump, provide interior heating by cooling down the nearby outside air temperature. An electric heater inside a building will provide 1-unit of heat energy for every 1-unit of electrical energy it consumes. A heat pump cooling outside atmosphere that is above 10-deg C can transfer 3-units to 4-units of heat energy for every 1-unit of electric energy.

A heat pump that is connected to a closed-loop water pipeline with an indirect connection to seawater at 15-deg C could transfer 6-units to 8-units of heat energy into a building for every 1-unit of electrical consumption. It is technically possible to connect heat pumps in series in an arrangement known as cascade heat pumping that can achieve higher output temperatures. Such technology could assist in preheating incoming cool water destined for commercial size hot water tanks or geysers, using 1-unit of electric energy to transfer 2-units to 3-units of heat energy into the incoming cool water.

Winter Refrigeration:

During winter, there will be a need for several commercial establishments to operate commercial size refrigeration technology that could be connected to independent circuits of the closed-loop thermal pipeline distribution network. Some of the water-cooled refrigeration technology could be applied extract potable water from humid winter air and thereby consume far less electrical energy that by using air-cooled refrigeration technology along Cape Town’s Atlantic coast. Some commercial establishments could transfer the water-based heat rejected from large-scale refrigerators to heat pumps, to preheat incoming water destined to be further heated inside water tanks or geysers, using electric heat.

Seawater Heat Sink:

During summer weather, owners of many commercial buildings across the Greater Cape Town area seek to maintain cool interior temperatures through air conditioning. Except that the overwhelming majority of air conditioners across the Cape Town area use warm summer air to cool the coils of the air conditioners. Due to low density and low heat capacity of the warm air, fans expend large amount of energy pumping massive volumes of air across the air-conditioner coils. An equivalent cubic unit of volume of water has 3400-times the heat capacity. As part of a closed-loop distribution system requires minimal energy to pump.

An internal current circulates seawater inside False Bay while a northward flowing current moves seawater along Cape Town’s Atlantic coast and provides large heat sink that could provide much of the commercial refrigeration, commercial air-conditioning and water-from-air extraction requirements for Cape Town’s central business district. There may be scope to install closed-loop water lines inside large multi-unit residential complexes that could sustain energy efficient operation of refrigerators, freezer units and air conditioners. Singapore operates a district cooling system while several European cities operate district heating systems based on a distribution network of closed-loop and insulated pipes.

Victoria & Alfred (V&A) Waterfront:

The coastal seafloor off Cape Town’s V&A waterfront development area represents a potential location to install an undersea closed-loop water pipeline to sustain their seasonal heating and cooling requirements. The pipeline may be installed on a section of the harbour floor on the north side of the development or along the calm water (north) side of the harbour breakwater, where it would function as a heat exchanger. It would connect to a combination heat pump and air-conditioning unit located within the development, with insulated pipes extended to other buildings within the V&A area.

A water-from-air extraction technology connected to a cool water pipe could provide much potable water for a variety of applications within the V&A development. There may be potential for the combination of solar heating and cascade heat pumping to provide for some of the hot water requirements, including the possibility of a heated indoor swimming pool. V&A senior management may wish to examine the feasibility of a heat pump/air-conditioner system connected to a submerged closed-loop water pipeline that functions as a heat exchanger that either dumps heat into the sea or removes heat from it.

Duncan Dock and Foreshore:

Duncan Dock is located within close proximity to large commercial buildings located at Cape Town’s foreshore district. A small amount of shallow dredging could allow for the installation of a submerged, closed-loop water pipeline on to the seafloor of Duncan dock that could function as a heat exchanger that uses seawater in the dock as a thermal reservoir and as a heat sink. Such a closed-loop water pipeline could connect to air-conditioning, heat pump and water-from-air technology inside major commercial buildings at the foreshore, allowing them to reduce their electrical energy consumption.

Port of Cape Town would need to advise in matters that pertain to the prospect of a water pipeline being placed on the harbour floor of Duncan Dock. A positive response would provide the basis for large buildings located in the foreshore area to reduce electrical energy consumption related to interior heating and cooling of the buildings. Connecting building heating and cooling to a closed-loop undersea water pipeline could reduce environmentally related electric energy consumption in these buildings by 50% to 70% compared to air-cooled air-conditioners and air-heated heat pumps.

Precedents:

There are several precedents internationally that involve the use of water to sustain the energy efficient operation of air-conditioners, refrigeration systems and heat pumps. In most cases, groundwater in porous rock serves to sustain such operation on a seasonal basis. During cold winter weather, groundwater as cool as 7-deg C serves as the thermal reservoir for heat pumps that sustain warm temperature levels inside buildings. During warm summer weather, the groundwater serves as the heat sink for centralized air-conditioning and commercial-scale refrigeration operations. A source of ample water can serve as either heat sink or thermal reservoir.

Overseas in Ottawa, Canada, low-grade geothermal energy found in groundwater sustains the seasonal heating and cooling requirements sprawling campus of Carleton University that has an enrolment of some 50,000-students. Summertime temperatures can exceed 30-deg C while winter temperatures drop to below the freezing point of water. Heat pumps connected to groundwater at 7-deg C sustains comfortable temperatures of 22-deg C inside the campus buildings. Both Carleton University and University of Cape Town have faculties of engineering that could communicate with each other to exchange information that pertains to the operation of using water to operate heat pumps and air-conditioners.

Conclusions:

The use of water to improve energy usage in air-conditioners and heat pumps is widely proven internationally. It is technically possible to install closed-loop undersea pipelines off the coast of the Greater Cape Town area and operate the water-filled pipelines as heat exchangers that either transfer heat from air conditioners into the seawater, or use seawater as a source of heat for heat pumps. Increasing the number of Cape Town area commercial-scale air-conditioners and commercial-scale refrigerators that are cooled by water instead of air would greatly reduce their energy consumption.

Some modern air-conditioners can operate in reverse mode as heat pumps that could transfer heat from cool outside air into buildings for interior heating. Alternatively, the heat pumps would consume less energy transferring heat from an ample source of water. Both technologies would consume far less energy than electric heaters that convert each 1-unit of electrical energy into 1-unit of heat energy. Using coastal seawater as a heat sink and thermal reservoir could make more efficient use of available electrical energy across the Cape Town area. Further research by UCT and by CPUT may verify future improvements in energy utilization.

About the Author:

Harry Valentine was born at Cape Town and attended primary and secondary schools in Cape Town’s District-6 area. He achieved his degree in engineering overseas at Carleton University.

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