12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, EstoniaDISCUSSIONFocused on level of users comfort and proper coolingof DH water during idling, time delay of LEDHsubstation to supply DHW with temperature 42 °C and47 °C was measured. Three different control strategiesrelated to tap delay were investigated. Obtainedresults represent case of IHEU used in single-familyhouse in period when space heating is not inoperation. Explored concepts can be evaluated fromtwo different points of view, due to highest advantagesfor customer and for DHN.The solution without by-pass is from energy savingspoint of view very interesting because doesn‘t needany DH water for idling, but from users comfort point ofview is very poor because of reduced comfort andproblems with wasting of water during waiting for DHWwith desired temperature. Solution without by-pass canbe probably used for substations equipped withcombined thermostatic and proportional flow controller,for customers with short branch pipes or for customerswith low requirements for level of users comfort. Ifsolution without by-pass will be used for substationcontrolled only with proportional flow controller, eventransport delay in 10 m long branch pipe for nominalflow for basin will be 32 sec. For period when spaceheating is operated, branch pipe will be kept warmfrom flow needed for space heating and time delay forsolution without by-pass will be very similar to solutionwith external by-pass. Anyway, in non-circularlyshaped DH networks, by-pass should be installed atleast at the end of a street, so it is better to findsolution how to use by-pass flow in useful way thansent it directly back to DH return. Considering this, it issuggested to use by-pass flow for whole yearoperation of floor heating in bathrooms to increasecomfort for customers and at the same time solveproblem with by-pass flow which otherwise increasingreturn temperature to DH network.From user comfort point of view, better solution thansolution without by-pass, but consuming more energy,is substation equipped with external by-pass. Bycomparison of results of concepts without by-pass(case 1) and solution with external by-pass, for casewhen tapping is performed after long period of idlingjust before by-pass opens again (case 2), we can seethat time delays are almost the same (see Table 4).Difference is only that for external by-pass are pipes inDH substation kept on higher temperature and it madeslightly faster reaction. In the case 3, time delay iseven more reduced since pipes in substation werewarmer by just finished by-pass flow. For controlconcept with external by-pass and tapping repeated5 minutes after previous one, time delay is againreduced, since HEX is still hot from previous tapping.The time delay for case 4 and 5 are almost the same,only difference is that in case 5 (internal by-pass), tapdelay is again reduced because tapping wasperformed 3 minutes after previous (to preventinfluence of by-pass) and thus HEX was warmer.If the requirement is to fulfil 10 sec tap delay for lessfavourable fixture, i.e. in our case basin (see Table 1),DHW should leave DH substation with temperature42 °C in 4 sec after tapping was started, because it willtake 6 second to reach the tap. This requirement wasreached only by concept with internal by-pass and onlywhen by-pass was already opened. On the other handfrom Table 3can be seen, that even for concept withexternal by-pass and tapping after long idling and justbefore expected bypass opening, DHW at atemperature 26 °C leaving substation in 3 sec. DHWwith this temperature is not sufficient for taking acomfortable shower for which temperature 37±1 °C ispreferred, but for washing hands this temperatureshould be enough. The values in Table 3 are for flowrate used for shower, but it can be used to explain thatit is time to rethink the suggested value of tap delayfrom 10 sec to another value and consider alsonominal flows and use of tapped water. The differentstandards for the different use of DHW based on newsolutions in DHW supply systems and results from testpanels are needed, because it may have someinfluence on design of optimized DHW systems.Nevertheless, for customers requiring DHW in veryshort time e.g. continuously or discontinuously (onlyduring rush hours) operated trace heating elementscan assure almost no tap delay by keeping DHWstaying in pipes on desired temperature.CONCLUSIONBased on literature study it can be concluded thathygienic requirement of DHW with 50 °C on outlet ofDHW heater is not needed for systems with a totalvolume of the DHW lower than 3 L.From results of our measurements and evaluation ofIHEU supplied by LEDH, only substation with externalby-pass with set point 46 °C is able to produce 47 °CDHW in time bellow 10 sec. The easiest step how todecrease waiting time also for other concepts is toinsulate HEX. This measure will reduce time delay forDHW tapping and also will decrease heat losses fromDH substation. The lower waiting times for DHW canbe also achieved by further optimisation of HEX in wayof decreased number of plates reducing volume ofwater in HEX and thus transport delay, and byincreased thermal efficiency of HEX (followed on theother hand by higher pressure loss). Thesemodifications can lead for higher temperature of DHwater returning to DH network, but during all ourexperiments, average return temperature was below20 °C, what is 5 °C less than is designed for LEDH.67