from a 2 - year study with measurements on a low - temperature DH system for low energy buildings

MAKING MODERN LIVING POSSIBLE Technical paper Results and experiences from a 2 - year study with measurements on a low - temperature DH system for low energy buildings Christian Holm Christiansen 1, Alessandro Dalla Rosa 2, Marek Brand 2, Peter Kaarup Olsen 3, Jan Eric Thorsen 4 DHC13, the 13 th International Symposium on District Heating and Cooling September 3 rd to September 4 th, 2012, Copenhagen, Denmark 1 Danish Technological Institute, Gregersensvej 1, DK-2630 Taastrup, Denmark 2 Technical University of Denmark, Dept. of Civil Engineering, Brovej, DK- 2800 Kgs. Lyngby, Denmark 3 COWI A/S, Parallelvej 2, 2800 DK-Kgs. Lyngby, Denmark 4 Danfoss District Energy, Nordborg, Denmark, +45 7488 4494, jet@danfoss.com districtenergy.danfoss.com

TECHNICAL PAPER Results and experiences from a 2 - year study with measurements on a low - temperature DH system for low - energy buildings A new low-temperature district heating system for low-energy buildings that operates with supply temperatures slightly above 50 C was presented at the 11 th International Symposium of District Heating and Cooling in 2008; the design includes newly developed substations and efficient distribution pipes, resulting in reduction of heat losses up to 75 % compared to traditional layouts. Since then, the first area using the new system has successfully been put in operation. This paper presents the results of a 2 - year study with detailed measurements of a low heat density area with 40 low - energy terraced houses in Denmark. The investigations include the determination of the heat losses from the distribution network, the pumping electricity consumption, the user behavior in terms of indoor temperature and domestic hot water consumption as well as detailed simultaneity factors to be used for network design. Moreover, the paper presents solutions for using the return water of existing networks to supply district heating to newly built areas and summarizes in general on how to integrate low - energy houses and district heating systems. Finally, it points to the potential of integrating low - temperature district heating systems in existing buildings as an effective solution towards energy - sustainability in the heating sector. Author(s) Jan Eric Thorsen, Danfoss District Energy, Nordborg, Denmark, +45 7488 4494 jet@danfoss.com Christian Holm Christiansen, Danish Technological Institute, Gregersensvej 1, DK-2630 Taastrup, Denmark Alessandro Dalla Rosa, Marek Brand, Technical University of Denmark, Dept. of Civil Engineering, Brovej, DK- 2800 Kgs. Lyngby, Denmark Peter Kaarup Olsen, COWI A/S, Parallelvej 2, 2800 DK-Kgs. Lyngby, Denmark Introduction An innovative low-temperature District Heating ( DH ) system for low - energy buildings that operates with supply temperatures slightly above 50 C has been successfully put in operation in 2010. This paper presents the results of a 2 - year study with detailed measurements of a low heat density area with 40 low - energy terraced houses and a communal building in Lystrup, Denmark, see table 1 and figure 1. The project dealt with the integration of sustainable solutions both for the end-user side and the energy supply side and aimed to: Demonstrate the operation and energy demand of DH applied to low - energy buildings and that the heat loss in the network can be maintained below 15 20 % of the total delivered heat. Project information owner Housing association BF Ringgården year of construction 2008 2010 site area [ ha ] 1.7 building units ( residential ) 40 terraced houses residents Seniors, young families number of residents 92 ( estimated ) building units ( teritary ) 1 communal building heated area [ m 2 ] 4115 plot ratio 1 0.24 1 built floor area / site area TABLE 1: Basic information on the project 2 Danfoss District Energy

Evaluate the simultaneity of the heat demand in case of low - energy buildings. Test two designs of low - temperature DH substations. Evaluate the user behavior in terms of indoor temperature and Domestic Hot Water ( DHW ) consumption. Technical description Heat demand The Danish Building Regulation 2008, later superseded by the Building Regulation 2010, set the maximum building primary energy demand for new constructions at the time of the project implementation. There were separate targets for residential building ( Space Heating ( SH ), DHW and the electricity use to the related installations, but not including lighting ) and non - residential buildings ( including lighting ). The requirement in residential building was defined as follows: E = 70 + 2200 /A [kwh / (m² yr)] (1) where E is the maximum annual primary energy demand and A is the gross heated area [ m 2 ]. The energy requirements also include two classes of low - energy buildings, which energy demand limit is calculated as follow: Low-energy class 1: E = 35 + 1100 /A [kwh / (m² yr)] (2) Low-energy class 2: E = 50 + 1600 /A [kwh / (m² yr)] (3) Primary energy factors of 2.5 for electricity and 1.0 for fuels and district heating are used. The settlement in Lystrup is designed for low - energy class 1. The design primary energy use for SH in the houses was 30 kwh /(m 2 year), see table 2. The insulation thickness of the building envelope is as follows: roof, 450 mm; external walls, 335 mm. The U - value of the windows are 1.1 W /(m 2 K). The layout of dwellings consists of seven blocks of houses, divided in 2 size categories: size C1 ( 87 m 2 ) and size C2 ( 110 m 2 ), see table 3. Heat demand The building installations, in terms of heating system, consist of a combination of radiators based on design supply/ return / room temperature of 55 / 25 / 20 C and floor heating in the bathroom. The DHW is prepared by one of the low - tem- FIGURE 1: The terraced houses and their spatial layout FIGURE 2: Sketch of the DHSU and IHEU principles of DHW production Specific heat demand ( design ) [ kwh th /( m 2 yr )] Specific SH demand 30 Specific DHW demand 13.1* total 43.1 * based on annual DHW use of 250 liter/m 2 and T = 45 C and design indoor temperature of 20 C as used in the Danish reference software Be06 TABLE 2: DH supply DH return DH supply DH return thermostatic bypass 120 l Building regulation design values DHSU principle DHW DCW IHEU principle DHW DCW Danfoss District Energy 3

perature DHW systems described in [ 1 ], [ 2 ]: the low - temperature Instantaneous Heat Exchanger Unit ( IHEU ) and the lowtemperature District Heating Storage Unit ( DHSU ), see figure 2. The layouts of the DHW distribution pipes and the floor plan of the dwellings were carefully designed, so that there is a separate pipe supplying each DHW fixture and the length of the pipe is minimized, see figure 3. Consequently, the water content in each DHW supply line, including the volume in the secondary side of the DHW heat exchanger, is kept to a minimum and it is below 3 liter: this is the maximum allowable water content that assures safety in relation to the Legionella risk, even without any treatments ( thermal, UV - rays or chemical ), according to the German guidelines for DHW systems ( DVGW, W551 ). block number total size [ m 2 ] number of dwellings type c1 type c2 1 771 5 3 2 727 2 5 3 594 3 3 4 528 1 4 5* 479 1 2 6 484 3 2 7 532 6 0 * including the communal building, A = 170 m 2 TABLE 3: Type and floor area of the buildings Heat distribution network A sketch of the DH network with the location of flow meters for monitoring is seen in figure 4. Besides the normal end - user heat meters and the main meter placed at the pumping station two additional meters are placed at the end of two different streets. One meter is measuring a part of the network with 11 DHSU's, the other is measuring on a network part with 11 IHEU's. substation DHW pipe Network dimensioning The network consists of flexible plastic twin pipes for dimensions up to DN32 and of steel twin pipes for larger dimensions. Heat loss coefficients are calculated according to [ 2 ] and pipe manufacturer data, figure 4. The other assumptions for the design were: Maximum pressure level: 10 bar. It is reasonable to design the network according to the maximum hydraulic load that can be withstood by the distribution pipeline; in this case the limit is drawn by the plastic service pipes, which requires pressure levels below 10 bar. In fact the pipeline systems must by regulations withstand pressures 1.2 1.5 times the nominal value. Thermostatic by-pass valves of IHEU's set to 40 C, in the customer's substation at the end of each street line FIGURE 3: Sketch of the floor plans with the layout of the DHW distribution pipelines. Type C1 ( left ), type C2 ( right ) Instantaneous Exchanger Unit (IHEU) flow rate meter: 0.01 0.38 m 3 / h flow rate meter (street): 0.26 3.4 m 3 / h Pumping station flow rate meter: 0.8 12 m 3 / h District Heating Storage Unit (DHSU) flow rate meter: 0.01 0.09 m 3 / h FIGURE 4: Sketch of the low - temperature network with the location of the meters. DH is delivered from the utility Lystrup Fjernvarmeto the pumping station 4 Danfoss District Energy

inner diameter [ mm ] [ W /( m K )] estimated cost in 2010 [ /m ] roughness lenght U [ mm ] [ m ] 11 =U 22 U 12 =U 21 purchase total 1 Alx 14/14-110 10 0.05 0.035 0.02 123 47 162 Alx 20/20-110 15 0.065 0.037 0.02 221 56 166 Alx 26/26-125 20 0.071 0.049 0.02 155 67 207 Alx 32/32-125 26 0.088 0.053 0.02 130 78 211 Tws - DN 32 37.2 0.085 0.056 0.1 90 82 240 Tws - DN 40 43.1 0.099 0.053 0.1 32 88 246 Tws - DN 50 54.5 0.096 0.06 0.1 16 122 268 TABLE 4: Pipe specifications. Alx: Aluflex twin pipes; Tws: Steel twin pipes, series 2, diffusion barrier at the outer casing. and set to 35 C, in all the other customers' substations. Design supply temperature from the mixing shunt: 55 C; design return temperature: 25 C. Maximum water velocity: 2.0 m / s; also in branch pipes. The simultaneity factor was assumed to be 1.0 in case of DHSU, due to the low semi - constant flow the unit was designed for. The simultaneity factor for the IHEU was the traditional consumer dependent approach used in Denmark. Design loads are for DHSU: 3 kw, for IHEU: 32,3 kw. Minimum supply / return pressure difference at the end - user's substation: 0.3 bar. Heat sources The distribution network in this case study is a typical example of how a lowtemperature DH scheme can be integrated in an existing network that has higher operating temperature. There are no heat sources on the site. The heat is provided directly from the medium - temperature DH utility Lystrup Fjernvarme. A pumping station and a mixing shunt are placed in the communal house. The pump is operated based on a pressure difference sensor placed at the critical point in the network. The mixing shunt is controlled by a return valve and a temperature sensor in the main supply pipe to the low-temperature network. The system is seen in Figure 5 together with the pressure line drawn from the pumping station to the end - user. Monitoring An extensive monitoring program and data acquisition system was established; the measurements presented here are mainly from the first monitoring period conducted during the weeks 26 47, 2010 with the main meters and individual meters in 22 dwellings in place; in these dwellings both DH meter, meter for DHW and a room temperature sensor were installed. Late 2010, DH meters in the remaining 19 dwellings were connected and the monitoring has continued ever since and will go on until end of 2013. Space heating and domestic hot water Based on the measurements in the first monitoring period, a heat load vs outdoor temperature curve was established, see figure 6. For IHEU the standby heat loss is about 25 W; for DHSU 80 W. It is seen that the average heat load during summer ( week 26 38 ) is higher than the standby losses which means that there has also been a SH demand in this period for some houses e.g. for floor heating of the bathroom. Based on the curve, the SH demand per dwelling was estimated to 5.1 MWh for the Danish reference year corresponding to about 51 kwh /( m 2 yr ), 70 % higher SH demand than building regulation design value ( see table 3 ). It was not the purpose of the project group to look at the building as- built vs. designed. Analysis of the measurements also rather indicates that the reason should be found in user behavior. Floor heating during summer is a parameter that is not taken into account in the building design values. Further, the indoor temperature, as measured in the living room, was 2 4 C above the design indoor temperature of 20 C during the heating season, see figure 7. In [ 1 ] it was shown that 1 C higher than expected room temperature can lead to 20 % higher SH demand in low - energy houses, so the room temperature alone, almost explains the high consumption. DHW consumption was measured to be 65 liter /( day house) in average. It is a low value, which is partly related to the number of occupants and their composition. Based on an estimate of the number of residents in the dwellings, it is assessed that the DHW use was equivalent to approx. 28 liter /( day person ). It should be noticed that the average cold water temperature was approx. 15 C and the average DHW temperature was 40 45 C, giving an average temperature difference of 25 30 C in the first monitoring period. According to [ 8 ], DHW use of 30 40 liter /(day person) and a temperature difference of approx. 40 C are typical values for Denmark. In the case study this would give an expected heat demand for DHW of 12 16 kwh /( m 2 yr ). In average the DHW consumption measured was 8 kwh/( m 2 yr ), which is less than the design value ( table 2 ). When added to the SH demand a total annual DH consumption of 5.8 MWh per house was found. Monitoring also demonstrated that DHW can be produced at temperatures of just 3 C below the primary supply temperature, e.g. 47 C at a DH supply temperature of 50 C as expected with the used substations. Danfoss District Energy 5

Operating temeratures Lystrup Fjernvarme that supplies heat to the new low - temperature area is a medium - temperature DH system. DH is supplied with up to 80 C during winter and down to 60 C during summer. In figure 8, the average weekly supply and return temperatures and heat load are seen for the 2 year monitoring period together with the shunted supply temperature. The maximum monitored heat load is 161.3 kw compared to maximum weekly average of 87.4 kw. The figure shows how the shunt has been adjusted during the period in order to get the low mixed supply temperature of just slightly above 50 C. Further the result of troubleshooting in individual building installations has secured a low return temperature. This is expected to be even lower after the local boiler man beginning 2012 has been provided a tool that updates him every week with return temperature and other relevant data of the individual houses. Based on the tool, he can guide the users towards better operation of the substations which has already been beneficial. In the first monitoring period of 2010 the two different substation types were compared specifically. In the 11 homes with DHSU, the average return temperature was 39.4 C in the weeks 26 47; in summer weeks 26 38 the average return temperature was 43.6 C. The high return temperature was primarily due to the malfunction of a single unit. The best performing DHSU registered a return temperature of 29 C in summer. The 11 homes with IHEU, the average return temperature was 34.7 C in the weeks 26 47; in summer weeks 26 38 the average return temperature was 40.3 C. The high return temperature was primarily due to 2 substations, where the control valves were defected and allowed a relative large amount of water to flow uncooled to the return pipe. The best performing IHEU registered a return temperature of 26 C in summer. Observing the same weeks, a year later, showed improved results as seen in figure 9 for the 11 IHEU's, even though the supply temperature had been reduced in the meantime. In general, the return temperature in the heating season ( week 39 47 ) was lower than during the summer, which confirms that the radiators delivers low return temperatures ( 28 33 C ). This occurred 2.2 bar 1.6 bar 65 C 6.5 bar 55 C 25 C dp=0.3bar elevation = 53 m AMSL 0 50 100 100 150 200 250 300 distance [m] To the main pipe (Lystrup district heating) 65 C Ringgården pumping station Shunt Return valve (close / half open) Ringgården low - energy network Thermostatic by-pass at each street end Consumer unit FIGURE 5: Simplified pressure line / temperature diagram of the mixing shunt during typical operating conditions Heat load for space heating incl. installation heat losses per house [W] 1500 1400 1200 1000 800 600 400 200 Week 38 47 (y = 85.0 x + 1168 [W], R 2 = 0.95) Week 26 38 (108 [W]) 0-5 0 5 10 15 20 Outdoor temperature [ C] FIGURE 6: Heat load vs. outdoor temperature curve based on average of 22 houses monitored during the first monitoring period, week 26 38 ( summer ); week 38 47 ( heating season ) 6 Danfoss District Energy

although the indoor temperatures during operation were some degrees higher than the design conditions ( 20 C ), which increased the minimum achievable return temperature from the radiators. Overall, the demonstration project has shown that the concept works, and that is further confirmed by the fact that there were no complaints from residents about the lack and quality of SH / DHW. Simultaneity factors at the far end of the DH net. This will statistically not be the case, why this puts the suggested curves to the conservative side. The DHW heat power, e(n), of one consumer was determined to be 4.7 kw for the DHSU case and 24.3 kw for the consumers with IHEU. The e(1) value for the DHSU case is a bit higher than the expected design value of approx. 3 kw. The explanation is the accuracy of the setting of the flow controlled motor valve, which controls the charging flow to the storage tank. The real charging flow is thus higher than the design flow. By readjusting this, a lower e(1) can be obtained. The parameter e(1) for the case with IHEU is lower than what is usually used by the designers in similar conditions, e.g. 32.3 kw in Denmark. On one hand, this result must be seen in relation to the housing type and residents behaviours ( mostly senior citizens and young families ). On the other hand, the analysis points at the fact that the dimensioning In order to define design loads in areas with low - energy buildings, simultaneity curves were developed based on monitoring data sampled every 4 minutes during the summer weeks 24 38 in the year 2010. Totally 38,000 data sets, or time stamps, were recorded for each of 10 IHEU's and 10 DHSU's. The methodology used to develop the curves is: Data is sorted in the way that the combination of the highest group load E(N) is calculated for each time stamp. To avoid too high simultaneity curves, it is accepted to exceed the suggested design load pr. consumer e(n) in 1 % of the tapping time. This is equal to shortcomings for a time period of 15 minutes if assumed the tapping is occurring all 24h / day. In practice the period will be quite shorter than 15 minutes. On the other hand the analysis assumes that the consumers with the highest DHW load by default are placed mean weekly temperature [ C] 30 25 20 15 10 5 0 Indoor temperature Outdoor temperature -5 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 FIGURE 7: Mean weekly outdoor temperature and indoor temperature in the buildings equipped with room temperature sensors in the living room week temperature, weekly average [ C ] 80 70 60 50 40 30 20 10 0 240 210 180 150 120 90 60 30 0 heat load, weekly average [ kw ] 26, 2010 2011 25, 2012 Week, year T supply shunt, low temperature area T supply DH T return DH Heat load FIGURE 8: Average weekly supply, return, shunted supply temperatures and heat load for the 2 year monitoring period Danfoss District Energy 7

of DH systems need a better basis for simultaneity factors, and that in future a greater consideration must be given to the installations types for the calculation of the optimal size of the heat distribution system. In figure 10, the developed simultaneity curves are compared with others in the literature. It can be seen for the IHEU tapping ( curve 11 ) that simultaneity is evident for up to two consumers. For n higher than 2 the e(n) value drops by the slope of approx. 1/n, which means that the following consumers are not adding any simultaneous load to the network. The initial design of the DH network is thus over dimensioned when compared to the actual DHW load. The initial design simultaneity factor is similar to curve 1 in figure 10. Looking at the DHSU ( curve e ) then charging simultaneity is found up to approx. 5 consumers. For a higher number of consumers also here the slope is approx. 1/n, which again means that the following consumers are not adding any simultaneous load to the network. Also here the initial DH temperatures, average week 26 47 [ C ] 60 55 50 45 40 35 30 25 20 15 10 5 0 DH supply 11 IHEU's DH return 2010 2011 FIGURE 9: Supply and return temperatures for 11 IHEU's, average of week 26 47, 2010 / 2011 100.0 10.0 4 7 Heating power demand for DHW, including simulteneity factors 5 6 Heat exchanger unit 1) V staabi, 32 kw 2) H formel, e oo = 1.19 32 kw 3) H formel, e oo = 0.70 32 kw 4) Poulsen, 37 kw 5) Po. Snit, 32 kw 6) Poulsen, 26 kw 7) Brydow, 20 kw 8) Lawaets, 32 kw 9) SE, A = 3.1, 34 kw 10) EHP, A = 3.1, 34 kw 11) IHEU Lystrup, e(1), 1% = 24.3 kw Storage Tank Unit (secondary side) a) V staabi, 13 kw b) H formel, e oo = 1.19 12 kw c) H formel, e oo = 0.70 12 kw d) Lawaets, 12 kw Storage Tank Unit (primary side) e) DHSU Lystrup, e(1), 1% = 4.7 kw e ( N ) [ kw ] 9 2 b 1.0 e 11 a 1 10 3 c 8 d 0.1 1 10 100 1000 N FIGURE 10: Comparison among simultaneity factors in the literature and the values derived from the measurements in Lystrup 8 Danfoss District Energy

design assumption of e(n) = 1 is leading to an oversize of the DH net. Anyhow, since the smallest available DH branch pipe dimensions are installed, this has no practical impact on the branch pipes. Additional information's for figure 10 can be found in [ 10 ]. Distribution heat losses and pump electricity consumption For a Danish reference year DH demand, heat loss in the distribution network and the annual electricity use of the pump were calculated based on duration curves divided in 8 representative intervals combined with load vs. temperature curves derived for the first monitoring period of 2010 ( as figure 6 ). In addition, a full year ( 2011 ) of measurements is available, see table 4. The heat loss of the reference year for the entire network is in line with the expected heat loss calculated in the design phase and comparable with the present share of the heat loss in the existing city - wide distribution networks in Denmark. However, the measured heat loss for the full year 2011 is about 11 % higher, which can be explained by a distance of unintended non - insulated pipes before the DH meter in each dwelling with IHEU. Considering these pipes insulated, the distribution heat loss for a network with 11 IHEU's is only slightly above the corresponding distribution heat losses for the 11 DHSU's. The total distribution heat loss in the low-temperature network are approx. ¼ of the estimated heat loss in the case of conventional medium - temperature network ( single pipes, series 1, 80 / 40 C, 6 bar system, 1 m / s flow velocity ). The electricity use for pumping was estimated to be 2,600 kwh / yr, equivalent to 9 kwhel /MWhth. This is comparable with the electricity demand for pumping purposes in existing well - established systems [ 4 ]. According to the design method, it was expected to measure a larger pumping demand; the lower electricity use for the pump is explained in practice by the fact that the pressure levels in the network were still well below the limits set. This points that there is room for optimizing the network design method even more, so that the heat loss can be significantly decreased, at expenses of an additional, but less significant from the overall primary energy point of view, pumping demand. Hydraulic limitations and noise must not be forgotten, though. Costs The total investment of the system has been estimated to 346,900 or approx. 8,460 per house, see table 5 [ 4 ]. It is also seen from the table, that the DHSU is about 30 % more expensive than the IHEU. Ongoing R & D project The investigations continue in an ongoing R & D project where low - temperature district heating is demonstrated in two existing single - family house neighbourhoods. The first area is in Tilst near Århus, where the low - temperature concept is being tested in a street with 8 houses with radiators. The focus is on strategies to prepare DK ref ( calc. ) year 2011 ( meas. ) total heat delivered to LTDH network MWh 287.2 273.9 heat demand MWh 238.1 219.4 distribution heat loss MWh 49.1 54.5 % 17.1 19.9 heat power, yearly avg. kw - 31.3 supply temperature, DH C - 67.4 supply temperature, LTDH C 55 52.7 return temperature DH C 30 34.1 electricity use, pumping station kwh 2600 2566 TABLE 4: Key data of network operation costs ( 2010 ) item [ / m ] [ / unit ] total [ ] pipes* 120 65,000 pipe fittings* 32 17,000 pipe laying** 131 100,500 DHSU substation* 3,700 41,000 IHEU substation* 2,600 78,000 substation installation** 1,000 41,000 pump + frequency controller* 2,400 + 2,000 4,400 total cost 346,900 cost per house 8,460 TABLE 5: investment costs the end - users and their installations for low - temperature district heating; strategies than can be used in a nearby area with 1000 single family houses. The second area is in Høje Taastrup near Copenhagen, where a low - temperature DH system supplying 75 houses with floor heating has been built. This area is supplied with an alternative mixing system as described in [ 9 ]: a 3 - pipe shunt arrangement is connected to the pumping station supplying mainly DH return water to the low - temperature area. When the return temperature is not sufficient, a portion of water from the supply pipe can be added into the mixing shunt. In this case the low - temperature network is supplied by water mixed from the supply pipe and the return Danfoss District Energy 9

pipe of the main district heating network. This solution can be installed in an existing district heating network at a location having a sufficient flow in the return pipe. In addition the monitoring continues in Lystrup with further analysis of substations and distribution network. Conclusions The demonstration project of a low - temperature DH network for low - energy buildings has shown that the concept works. The results show that it is possible to supply the customers with a supply temperature of approx. 50 C and satisfy both the SH requirements and the safe provision of DHW. This fact is confirmed by the fact that there were no complaints from residents about the lack of SH or DHW. The energy efficiency target was met, being the distribution heat loss equal to 17 % of the total heat production for the Danish reference year. Even better real - life performance is expected when unintended non - insulated pipes are getting insulated. In DH networks of this kind, serving low heat density areas with no possibilities for future expansion, the design should envisage the exploitation of the maximum pressure that can be withstood by the media pipes. The network design method can thus be optimized, so that the distribution heat loss can decrease even further, at expenses of an additional, but less significant, pumping demand. The analysis points at the fact that the dimensioning of DH systems need a better basis for simultaneity factors and that a greater consideration must be given to the operation of the SH and DHW installations, for the calculation of the optimal size of the heat distribution system. The results demonstrate that it is possible to guarantee an energy - efficient operation, but it is very important to obtain proper functioning of each substation, otherwise unacceptable return temperatures result. In the case considered, the distribution heat loss for the area with DHSU's are slightly lower than in the area with IHEU's. The sum of the distribution heat loss and the standby heat loss from the substation is on the other hand larger in the DHSU case than in the case with IHEUs, because the additional heat loss due to the storage tanks more than counteracts the reduction of the distribution heat loss. However, in areas with hydraulic limitations, such as outer urban areas, DHSUs offer in turn some advantages, thanks to the lower peak pressure / load requirements. Moreover, the smallest media pipe diameters of the house connection pipes in the market have still a valuable water flow overcapacity and this suggest that smaller volume of the storage tank can be chosen, in case of DHSU, and this would reduce the substation heat loss, space occupation and costs somehow. The conclusion is that within the tested substations, the IHEU is a better solution in regards to energy performance, installation costs and space requirements. Anyhow there is no superior substation concept for all purposes, but the system should be chosen taking into account the specific characteristics of the site and of the demand. Aknowledgement The projects have received grants from the Danish EUDP - program making it possible to develop and demonstrate the low - temperature concept. 10 Danfoss District Energy

References More articles [1] Udvikling og demonstration af lavenergifjernvarme til lavenergibyggeri (Development and demon-stration of low energy district heating for low energy buildings, in Danish), Energistyrelsen, 2009. [2] CO 2 -reductions in low-energy buildings and communities by implementing low-temperature district heating systems. Demonstration cases in EnergyFlexHouse and housing association Boligforeningen Ringgården, Danish Energy Agency, 2011. [3] Wallenten P., Steady-state heat losses from insulated pipes, 1991, Lund Institute of Technology, Sweden. [4] Dalla Rosa, A., The Development of a New District Heating Concept. Network Design and Optimization for Integrating Energy Conservation and Renewable Energy Use in energy Sustainable Communities. PhD thesis; Technical University of Denmark, 2012. [5] Full-scale demonstration of low-temperature district heating in existing buildings, ongoing Danish EUDP project, 2012. [6] Olsen P.K., et. al., A new low-temperature district heating system for low-energy buildings, in the 11 th International Symposium on District Heating and Cooling, 2008, Reykjavik, Iceland. [7] Paulsen, O., et. al., Consumer unit for low energy district heating network, in the 11 th International Symposium on District Heating and Cooling, 2008, Reykjavik, Iceland [8] Brand, M, et. al., A direct heat exchanger unit used for domestic hot water supply in a singlefamily house supplied by low energy district heating, in the 12 th International Symposium on District Heating and Cooling, 2010, Tallinn, Estonia. [9] Christensen, S.K. et. al., New district heating concept: Use the return water for supply in new areas / networks [10] Thorsen, J.E. Cost considerations on Storage Tank versus Heat exchanger for htw preparation, The 10 th International Symposium on District Heating and Cooling 2006. Hannover, Germany. [1] Jan Eric Thorsen and Johnny Iversen, Impact of lowering dt for Heat Exchangers used in District Heating Systems, In proceedings of: 13 th International Symposium District Heating and Cooling, Copenhagen, Denmark, 3 rd 4 th of September, 2012. [2] Jan Eric Thorsen and Halldor Kristjansson, Cost Considerations on Storage Tank versus Heat Exchanger for Hot Water Preparation, In proceedings of: 10 th International Symposium District Heating and Cooling, Hanover, Germany, 3 rd 5 th of September, 2006. [3] Erika Zvingilaite, T. Ommen, B. Elmegaard and Martin Lyder Franck, Low Temperature District Heating Consumer Unit With Micro Heat Pump for Domestic Hot Water Preparation, In proceedings of: 13 th International Symposium District Heating and Cooling, Copenhagen, Denmark, 3 rd 4 th of September, 2012. [4] Halldor Kristjansson, Controls Providing Flexibility for the Consumer Increase Comfort and Save Energy, Published in: Hot & Cool, International magazine on district heating and cooling, 1/2008. [5] Jan Eric Thorsen, Analysis on flat station concept, In proceedings of: 12 th International Symposium District Heating and Cooling, Tallin, Estonia, 5 rd 7 th of September, 2010. [6] Oddgeir Gudmundsson and Jan Eric Thorsen, How to Realize Lower Temperatures in Existing DH Networks, Published in: Euro Heat & Power 4/2012, pp. 37 44, 2012. [7] Oddgeir Gudmundsson, Jan Eric Thorsen and Lipeng Zhang, Cost Analysis of District Heating Compared to its Competing Technologies, In proceedings of: 4 th International Conference on Energy and Sustainability, Bucharest, Romania, 19 th 21 st of June, 2013. [8] Oddgeir Gudmundsson and Jan Eric Thorsen, District Heating Application Handbook. Published by: Danfoss A/S District Energy Division. More information Find more information on Danfoss District Energy products and applications on our homepage: www.districtenergy.danfoss.com VF.HZ.C1.02 Produced by Danfoss A/S, DEN-SM/PL 12/2013