Heat Booster Substation for Domestic Hot water and Circulation Booster for Domestic Hot Water Circulation: Delivery no.: 10-1c

Jan Eric Thorsen, Torben Ommen, Wiebke Meesenburg, Kevin Smith, Eduard Melo Oliver

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Abstract

This report includes two new developed, installed and tested concepts as part of the Energylab Nordhavn project1:

1: A Heat Booster Substation (HBS) in Aarhusgade 140 - Havnehuset, a multifamily building with 22 individual flats, was installed in February 2018. The HBS is a district heating (DH) substation operating at an ultra-low temperature DH (ULTDH) supply temperature of 35-45°C. The concept includes a main electric heat pump, boosting the DH supply temperature to a level where domestic hot water (DHW) can be produced by means of an instantaneous heat exchanger. The HBS concept takes energy from the ULTDH supply and by the heat pump a part of the flow is cooled down by the evaporator and returned to the ULTDH return, and with this energy the other part of the flow is boosted by the condenser and led to the DH storage tank. Besides this, the HBS maintains the DHW circulation temperature by means of a small heat pump. No external heat source is needed as both heat pumps move the energy originating from the ULTDH supply. The DH storage tank is accumulating the energy, and this storage tank makes it possible to shift the charging time of the tank, independent on the DHW tapping occurrence. Hereby the service of load shift flexibility is provided in relation to electricity and DH. The heating service of the building is supplied with ULTDH as well but is not a part of this WP. The ULTDH is realised by a mixing-loop on building level.

Based on the experience made (12 months so far), it can be concluded that the HBS unit is successfully installed, tested and operating. The DHW is produced at 55°C, DHW circulation is raised continuously from 50-55°C, with a DH supply temperature of 45°C and a DH return temperature of typically 30°C. The share of electric energy consumption for DHW and DHW circulation service is 14%, at the average produced DHW volume of 1.700 liters pr. day. Whereas the DH share is the remaining 86%. Due to the variation of DHW draw off pr. day over the year, the electric share varies e.g. from 12%, at a produced DHW volume of 2.500 liters pr. day, to 17% at a produced DHW volume of 1.000 liters pr. day. Note that electric share is based on DHW as well as DHW circulation production.

The daily average DHW load shift potential is 75 kWh/day for the 22-flat building, hereof electricity accounts for 7 kWh/day and thus DH accounts for the remaining 67 kWh/day. On a yearly basis it’s at least on the same level as the load shift potential for the heating system of the building. Regarding capacity flexibility, this is 3 kW electric and 30 kW DH realized for e.g. a period of 1 hr. and 10 min. before the morning DHW peak and before the evening DHW peak in average over the year.

Regarding fuel shift flexibility, which is obtained by varying the evaporation temperature of the main heat pump, tested in the range corresponding to a DH return temperature from 20°C to 30°C, this result in 2 kWh pr. day, thus the electric consumption can vary from 6 to 8 kWh for an average day, with the corresponding DH variation from 66 to 68 kWh/day. The Fuel shift flexibility is regarded as minor.

A prognosis and economic based scheduling of the HBS charging is implemented, optimizing for lowest energy costs,latest possible charging among the lowest cost periods and observing the constrains of min. and max. charging level of the tank. By charging the tank as late as possible the heat loss from the tank is reduced due to a lower surface temperature.

A number of feasibility studies has been made for the HBS concept compared to the LTDH concept with different energy sources, showing under the current tariffs system the concept is very limited economically feasible, say only feasible under special system conditions. For a new urban development area, like Nordhavns Levantkaj, the HBS concept could become relevant. This type of area could be designed for ULTDH, where the DH energy input should be 6 the DH return water from the existing DH system. This temperature is typically in the range 35-45°C, and thus relevant for the HBS concept. Still adjustments to the current energy price structures will be needed to make the concept feasible. Also, where the source of energy for DH is at low temperature level, e.g. solar thermal, geothermal, industry surplus and data centres, the concept of HBS in combination with underfloor heating is relevant to consider. Further, the value of the load shift potential, temperature dependent DH energy prices both flow and return and the electric energy prices will determine if the HBS concept is feasible in each individual case.

2: A Circulation Booster (CB) is installed in Strandboulevarden 3, a multifamily building with 15spacious apartments.The aim was to secure a low DH return temperature from the DHW system, where especially the DHW circulation caused a high DH return temperature, due to the relative high heat loss of the DHW circulation system. The CB is boosting (heating) the DHW circulation water from 50°C to 55°C in two steps, by means of a direct heat exchange and a heat pump booster, using DH at normal temperatures (70 – 100°C) as the energy source. The DH return temperature is reduced from a level of approx. 45°C to approx. 20°C for the DHW service (DHW preparation and DHW circulation).

Based on the experience made (6 months so far), it can be concluded that the CB is successfully installed, tested and operating. The DHW circulation is boosted continuously from 50-55°C, with a DH supply temperature in the range from 70-100°C. The share of electric energy consumption for the CB concept is 17% at a representative DH flow temperature of 80°C. The representative DH return temperature from the CB is 23°C (not including the DHW storage tank DH return). Within the range of DH flow temperatures 70 – 100°C, the electric share is in the range 20- 15% of the energy needed to boost the DHW circulation. The remaining energy source is DH, with a share of 80 – 85%.

Load shift is not possible for this concept, since the CB is running continuously, and energy storage devices are not obvious for this concept.

Fuel shift is possible, to a minor degree, realized by influencing the condenser outlet temperature, and thus influencing the balance of heat from the condenser and the direct heat exchange. No tests are so far made in this regard.

A number of economic scenarios have been made, focussing on different bonus structures related to reduced DH return temperature and electric energy costs. The current tariff structure does not give a feasible economic case for the CB concept. But considering a more progressive bonus scheme for providing a low DH return temperature, the market potential could be interesting in the segment of existing buildings and partly also new buildings with DHW circulation systems. This is also supported by having the simple retrofit demands in mind and driven by the argument that a low DH return temperature is the precondition for a low DH flow temperature, which is one of the characteristics of the 4th generation DH concept.
Original languageEnglish
Number of pages34
Publication statusPublished - 2019

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