Retrofitting existing heating systems with water-water heat pumps fed by low temperature water networks
Marco Masoero, Chiara Silvi, Dipartimento Energia, Politecnico di Torino
Gianfranco Pellegrini, AREA Science Park, Trieste
The paper presents a water-water high temperature heat pump system, coupled to a district piping network distributing low temperature water. The heat pumps utilize, as low temperature heat source, water distributed with a pipework which is very similar to conventional district heating one. Similarly to district heating sub stations, the pipework feeds heat pumps installed in the building plant room, replacing the boilers which are part of the existing centralized heating and SHW production (if present) system. An application for the city of Torino is presented, including energy, environmental and economic analyses.
The present study concerns the application of high temperature (Tsupply ≥ 80°C) water-water heat pumps in an innovative district heating system. The heat pump use, as the centralized low temperature heat source, water distributed with a pipework which is similar to a conventional district heating one. Similarly to conventional district heating sub stations, the pipework feeds the heat pumps which are installed in the existing boiler rooms. Since the heat pumps are capable of producing water at temperature levels comparable with that of a standard combustion boiler, they can be easily coupled to conventional radiator heating systems and to centralized SHW production systems, if present.
The primary circuit of the system therefore consists of a district water distribution network feeding the urban buildings. Each building served by the system will be equipped with a centralized system consisting of a plate heat exchanger, whose primary side is connected to the district pipework, and secondary side to the evaporator of the high temperature heat pump; the condenser of the heat pump, in turns, will be connected to the mechanical and electrical services of the building
2. Description of the system
2.1. Low temperature heat source
It is necessary to identify a centralized low temperature heat source having enough capacity to satisfy the total demand of the evaporators of the connected heat pumps. Various sources may in principle be exploited: sea, lakes, rivers, canals, unused pits, shallow table water, urban or industrial water mains, sewage systems, etc.
The shallow water table solution implies digging a pit in which a submerged pump is installed. The pump must have a sufficient capacity to fulfill the demand of all the connected heat pumps assuming a water temperature drop in the primary side of the evaporators of 3°C to 5°C. The pressure head must be sufficient to offset the total geometric level difference and the friction losses in the pipework, evaporators, valves, etc.
The sea water table solution implies installing a submerged pump at a few meters depth with sufficient capacity to fulfill the demand of all the connected heat pumps, assuming a water temperature drop in the evaporators of 3°C to 5°C. The pressure head must be sufficient to offset the total geometric level difference and the friction losses in the pipework, evaporators, valves, etc. The sea water crosses the primary side of a titanium heat exchanger. The distribution pipework is connected to the secondary side of the heat exchanger and feeds all the connected heat pumps. This solution must include all the accessories necessary to minimize the formation of scale, algae, corrosion, etc.
When the water works are used, a booster pump must be installed with sufficient capacity to fulfill the demand of all the connected heat pumps, assuming a water temperature drop in the evaporators of 3°C to 5°C. The pressure head must exceed the water mains pressure, in order to offset the friction pressure drops and allow re-immission. The water extracted from the mains flows into a heat exchanger of food processing grade and is then re-introduced in the mains at a slightly lower temperature. This solution implies the realization of hydraulic connections to the mains in correspondence of each building served.
2.2. District distribution pipework
The district distribution consists of pipework which is similar to a conventional district heating one, the only difference being that rather than pre-insulated pipes, uninsulated HDPE pipes would be used. Compared to conventional district heating, such pipework would be less invasive and significantly cheaper to install. The following are some advantages of this system:
- Installing the heat pumps close to the end users allows the distribution of low temperature water in the district pipework, thereby eliminating costly pre-insulated pipes and making the pipework construction easier;
- High initial investments for the heat production central plant are not necessary;
- Interventions are modular and may be scheduled in time according to the available financial resources;
- Costs of the infrastructure and pipework are much lower (uninsulated water mains pipes, simpler soldering procedures, smaller excavation sections, lower cost pipes and fittings, etc.);
- Plant operation and management costs are quite low;
- Malfunctioning of the central plant does not influence the end users;
- Negligible heat losses of the distribution network (this is particularly important for summer DHW production);
- High percentage of energy (at least two thirds from renewable sources).
Conventional district heating determines unquestionable advantages for the urban environment and microclimate, potential economic bonus for the end users, and energy benefits if recovered heat is exploited; the works for the installation of the district pipework, however, causes severe interference with the local traffic and occasional complaints from the population. Another problem is that emissions are not reduced if heat is produced by combustion rather than from a recovery process. Such problems are largely overcome with the proposed concept.
The construction of the distribution network is much simpler, faster and less invasive, with a significant reduction of inconvenience for the population. In addition, no central production plant is needed, and consequently there is no need to devise an energy recovery scheme. Furthermore, at least 70% of the energy is renewable (geothermal or hydrothermal).
The goal of this pilot project is to show the technical feasibility of initially exploiting all the existing urban infrastructures capable of delivering water (water mains, sewage systems, underground canals, unused pits, etc.) and successively plan to build new infrastructures for delivering water drawn from centralized sources (water table, rivers, lakes, sea, etc.). It is therefore a system which would allow the diffuse adoption of high temperature heat pumps in most urban contests.
Another possible solution foresees the exploitation of canals present in the city subsoil. Particularly, in Torino there are seven distinct infrastructures potentially suitable for this use. In this study, some hypotheses of exploiting sewage systems have been considered:
Sewage system of Piazza Sofia, Torino. This sewage system has a flow rate of 2 – 4 m3/s at a temperature of at least 28 – 30°C,; this allows to obtain very high efficiency values for the heat pumps, with considerable benefits in terms of energy and cost savings and emission reductions.
Sewage treatment system of Castiglione Torinese. The plant has a handling capacity of 225.000.000 m3/yr. In this case too, the water is warm and the heat pumps could be used to efficiently serve the residential buildings in the area nearby the plant. The sewage water cooled by the heat pumps would then be returned to the Po river at a slightly lower temperature, with evident environmental benefits, particularly in the winter period when the river low flow rates make it difficult to respect the discharge temperature limits set by legislation.
Finally, in the Torino area, in addition to the mentioned sources, the urban rivers Po, Ceronda, Stura, Dora Riparia and Sangone could be exploited with the same criteria.
2.3. Building substations
In the building boiler rooms the plate heat exchangers and the heat pumps are installed. Water form the centralized source is fed to the building with underground pipes reaching the boiler room.
It is possible to connect to a given source a maximum number of buildings until the available flow rate from that source is achieved. The water at the outlet of the heat exchanger is then discharged in the sewage system or in a surface water body, or pumped back to the source.
3. Economic, energy and environmental aspects
For sake of briefness, the analysis has been focused on the following case:
• Single size heat pumps of 100 kW output each;
• Existing boilers of equal size, using natural gas as fuel, serving identical buildings of 30.000 m3 heated volume;
Total cost has been broken down into three terms (Fig. 1):
• The cost of the district pipework is independent on the number of connected heat pumps and becomes almost negligible when the number of units exceeds 20;
• The cost of the connections, albeit variable, is virtually negligible;
• The main cost item is the installation of the heat pumps: this represents an incentive for the growth of the manufacturing companies involved in the project.
It can be noted (Fig. 2) that the achieved savings are significant: by installing 100 heat pumps of 100 kW output each, the yearly energy expense goes from 950 k€ (for boiler gas consumption) to 600 k€ (for heat pump electrical consumption). To be on the safe side, the cost analysis did not take into account operation and maintenance, which would in any case be more favorable for the heat pump system for the following reasons:
• Lower maintenance requirements;
• Less demanding requirements imposed by the safety and energy saving regulations.
In the examined case, the annual primary energy consumption diminishes from 1160 TOE to 870 TOE (Fig. 3). The emissions associated to the heat pumps are obviously the ones associated to the production of the electricity driven the heat pump by power plants burning fossil fuels. Local emissions are, on the contrary, eliminated. This aspect is worth stressing, since local emissions are much more detrimental to the urban microclimate then those produced in the power plant sites, which are selected according to environmental feasibility criteria. In the case study, the proposed solution yields an yearly emission reduction of CO2 from 2700 to 1850 ton/yr (Fig. 4) and of NOx from 3,5 to 2,8 ton/yr (Fig. 5).
The low temperature district heating concept allows a significant reduction of urban pollution due to building heating. It is sufficient to replace the boilers of conventional radiator hydronic systems without costly retrofit actions on the building or heating system. The first reference market is represented by existing urban building stock (residential, historical buildings, schools, hospitals, etc.) and by industrial / commercial ones. Nevertheless, the concept is of interest also for new installations, particularly when a low thermal capacity system heating is desirable.
The main advantages concern the economic, operational, energy and environmental aspects. The higher initial installation costs, as compared to conventional boilers, are offset, over the operational life of the system, by the savings on the annual energy costs (35-60%), but also on the reduction of operation and maintenance costs and obligations linked to safety regulations. In Italy, the payback time may be between 3 and 5 years, depending on energy rates and installation conditions.
From the energy standpoint, at least 70% of the energy delivered comes from renewable free sources, and, if the electricity used by the heat pumps is also produced mainly from renewables, this percentage becomes even higher. Finally, from the environmental viewpoint, zero local emissions contribute to an increased environmental quality of urban areas.
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