Waste heat recycling

Waste heat recycling

A new heat-recovery technology being developed to capture low-grade heat in wastewater shows commercial promise.

Low-grade heat 25ºC (80ºF) in wastewater discharged into the drainage system is an everyday by-product from commercial and domestic properties. It comes from sources such as washing machines, dishwashers, shower units, baths, sinks and commercial process plants.

In domestic terms, there are an estimated 58 million dishwashers, 232 million washing machines and 63 million showers units across Europe, with at least 90% of the input energy going into heating the water for these appliances. This equates to about 26,500 terajoules of thermal energy. The commercial sector is even bigger.

This source of energy from wastewater has been largely ignored, but the rising cost of energy has given the LowHeat project great importance in the future development of plumbing solutions. Legislation, the current environmental climate and public awareness have made LowHeat a viable commercial proposition.

The project evolved from an under-bath heat exchanger developed by Pera, an international network of technology development and industry support centres, under a cooperative research program funded by the European Commission (EC)

This project was called Warm-it, and the product is now near to market. However, Warm-it benefited showers only, by pre-heating the cold incoming mains to reduce energy consumption or to improve the shower performance without the need for upgrading electrical cabling.

This innovative product prompted the exploration of other warm wastewater applications, leading to a prototype that could recover energy from the whole of a commercial installation or domestic building.

The concept led to a European-funded project with a value of more than €2 million (US$3.9 million) that has a consortium of 12 small and medium-sized enterprises (SMEs) from six European Union (EU) countries. The project is being coordinated by the United Kingdom’s Institute of Plumbing and Heating Engineering (IPHE), the sector’s professional body, with research and development led by Pera.

The charter of IPHE and Pera was to develop a ‘proof of principle’ demonstrator achieving the objectives set out in the proposal. The basic objective was to try to recover an ambitious 50% of the energy from warm wastewater.

Preliminary testing was conducted to verify heat losses in wastewater from a variety of sources including washing machines, dishwashers, baths, washbasins and sinks.

The technical objective covered the following areas:

•wastewater temperature averages and ranges;
•flow rates;
•volumes of wastewater discharged;
•rinse cycles from washing appliances and dishwashers;
•quantity of debris/solid particles;
•the pH of the wastewater;
•corrosive properties of wastewater;
•foaming characteristics;
•generic locations of domestic appliances.

These tests confirmed that warm water is dumped to waste with a significant amount of available energy, but different design characteristics would have to be explored.

After considering many different drainage possibilities, the consortium agreed that utilization of conventional drainage systems and components would provide a greater variety of installation possibilities. Therefore, as a proof of principle demonstrator a common drainage fitting – the 110mm trapped gully – would be the agreed collection point for the warm wastewater. If energy can be extracted from a fitting holding only 3L of water, the potential for commercial applications would be proved.

Once this energy had been captured, where would it be put to best use? Many applications were considered, including solar heat stores, heat banks, underfloor heating systems, condensing boilers’ returns and as a pre-heat for hot water systems. As a proof of principle demonstrator, returning the energy to the hot water storage was considered appropriate, as this would need to account for the different types of hot water systems across Europe.

To ensure the device could effectively transfer heat into the hot water storage cylinder, verification was required to determine that typical cylinder temperatures would at some point allow good heat transfer. Research indicated that the section of water below the heat source (S6) is cold enough to enable a good temperature difference between the wastewater and the wholesome water, and therefore is a good potential for high heat transfer rates. The 3D graph shows the complete heat-up cycle from a cold cylinder to being brought up to normal operating temperature.

Early consideration was given to regulations, not only in the UK but across Europe. The development of the LowHeat device has brought together two different systems, wastewater and wholesome water, that are classed as fluid categories at opposite ends of the spectrum. Regulations, rightly so, do not allow these two categories to mix, therefore a ‘type A’ air gap is necessary, which requires a physical separation of the fluids. This separation has an inevitable effect on the heat transfer capabilities of the device, but consortium member CRS had the perfect answer in the heat pipe.

In their simplest forms heat pipes comprise a sealed vessel containing a working fluid and its vapor, together with a capillary wick lining system. A heat pipe can be thought of as a super heat conductor, providing a thermal absorption and transfer system with the capability of moving large amounts of power in the form of heat energy.

The application of heat at any point on the heat pipe surface causes a liquid/vapor phase change inside that enables heat energy to be transmitted in the vapor phase with only a minimal temperature gradient. In terms of thermal conductivity, a heat pipe can exhibit thermal performance able to exceed that of an equivalent-sized component made from pure copper by more than 1000 times.

For LowHeat, the heat pipes are produced in rod form with a circular cross-section similar to a gatling gun. Although the operational principle is simple, the technology required to produce heat pipes that perform optimally and reliably is complex and very exacting.

The above diagram shows views of a sectioned heat pipe metallic enclosure with the installed capillary lining. Operating on the principle of latent heat of vaporisation, heat pipes utilise the phase change of a working fluid operating in what is an otherwise completely evacuated and sealed enclosure.

The fluid actually exists within the vessel as a wet saturated vapor. When heat is applied to any point along the external surface, evaporation of the fluid occurs at this point within the heat pipe, then condensation occurs at any other internal point that is at a fractionally lower temperature. In so doing, the latent heat of vaporisation during the phase change process is utilised to effect a very efficient energy transfer.

The passive operation of heat pipes requires no external motive power except that of the applied temperature differential provided by the thermal loading. Having no moving parts, they are silent in operation, are extremely reliable, and provide a simple solution to the heat transfer from wastewater to wholesome water by providing the physical air break.

This early design evolved to ensure that the heat was instantly and directly dissipated into the hot water cylinder. Although the design was simple, required no moving parts and could be retrofitted, the performance of the Gatling gun was limited, due to the size of an existing immersion heater boss. The typical performance of the Gatling gun can be seen in the accompanying table.

Special consideration was given to the design and construction of the in-line heat exchanger. Simple decisions were made to help the overall design of the heat exchanger, including:

•counter-flow heat exchangers for higher performance compared with parallel-flow units;
•maximizing surface area to extract heat from the water;
•minimizing restriction of flow rates through the heat exchanger; and
•utilizing, where possible, standard plumbing fittings and connections.

The heat pipes allowed for a regulatory compliant device separated into two sections. The prototype was manufactured from the plastic material Delrin. To maximize the heat extracted from the water, we ensured that the flow was not restricted through the heat exchanger. Therefore, the CSA of the water around the heat pipe was the same as a UK standard plumbing pipe of 15mm (having an internal diameter of 13.6mm).

The first in-line heat exchanger was developed using 10 heat pipes. As each heat pipe is charged to perform to 300W, it was fair to assume that the heat exchanger could have a power performance of 3000W.

By using standard plumbing fittings to connect the two sections, we lost some of the useful length of heat pipe, resulting in a slight loss of performance. Nevertheless, with a wastewater temperature of 65°C, the heat exchanger’s power performance was 2200W.

This design enables the installation of the in-line heat exchanger to be application -specific, depending on size of installation and space. The latest tests have been carried out using a heat exchanger with 30 heat pipes. This had a power performance of almost 4500W and would be ideal for a small commercial premise.

The wastewater and the wholesome water are pumped around the system at a calculated optimum flow rate. As an energy-saving device, low power consumption is crucial. The pump that will be housed in the LowHeat drainage device is a low-power device using less than 100W of energy.

The wireless wall device was developed to enable the user to monitor how much energy is being saved with the LowHeat installed. The user has control over the functions of the device, which can be reset at any point in time and displays the following information:

•real-time performance of the heat exchanger;
•accumulated kWh;
•accumulated cost saving in euros;
•reset button.

The battery-operated wireless wall device is easy to install and uses a low-power IEEE802.15.4 radio to receive data up to 30m from the drain device.

When the different elements are brought together, a simple process of heat transfer takes place. The system becomes operational when the temperature of the warm wastewater reaches 30°C in the drain trap device. The pump then circulates the wastewater through a filter and into the heat exchanger. Simultaneously, the cool water from the bottom of the hot water storage cylinder is circulated to the wholesome side of the heat exchanger. The heat transfer takes place via the heat pipes and the free energy is transferred back into the cylinder in the optimum position.

This operation continues until the useful energy contained in the trap has been recovered and the system automatically switches off until the next batch of warm water is sensed.

As shown in the table below, the figures describe the percentage of heat recovered, calculated from the amount of energy available in the wastewater above 30°C.

The shower, for example, at 44°C with 4L/min flow rate recovers ~40% of energy, with the shower having more than ~2,000,000J available (0.53kWh) and recovering ~800,000J (0.22kWh).

The consortium has concluded that the ideal market for the LowHeat device would be in commercial applications such as processing plants, laundries and hotels that have a high hot-water demand and usually a plant room to accommodate
the required size of device.

For domestic applications, new buildings would be ideal, as the plumbing systems and space requirements can be easily planned for.

There is a retrofit market and the Gatling gun concept fits well into this area. Our Polish colleagues will soon be starting a program of refurbishing many properties, and they see this as their favored way forward.

Due to the potential of the device for energy savings, the EU chose LowHeat as one of five projects to be filmed as part of a promotional DVD. The DVD, which will be shown on European television stations, will promote funding opportunities available across Europe, helping SMEs to innovate and provide opportunities for business growth. The three days of filming took place at the end of 2006.

The project’s success led to selection as one of three finalists in the UK National Energy Efficiency Awards 2006 (pictured left), under the category of research and development.

By celebrating the achievements of the organizations helping to conserve energy resources, the event has helped to increase awareness, share good practice and demonstrate that energy efficiency can be easily achieved and cost-effective.
There was an overwhelming response to the awards, with more than 150 high-quality entries across the different categories. The research and development category focused on projects that could be shown to substantially increase the understanding of how we can reduce energy use for products, activities or services.

This technology report was compiled with the assistance of:

IPHE Technical Director & LowHeat project
co-ordinator dalec@iphe.org.uk
Darren Woodcock EngTechMIET CompIPHE
Pera Project Manager
For further updates visit www.lowheat.

Related Posts