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, Estoniaimpact of this solar radiation. Hence this sensor wasplaced in full view of the sun. The first day was sunnyduring most of the morning until midday, while thesecond day was cloudier.will lower the need of additional heating from theradiator system, by coordinating the thermal inertia ofthe building with freely available heat, e.g. heat fromsunlight or electrical appliances, to balance the heatingneed. This notion is supported by our results as wehave shown that the thermal inertia of even a small ormedium sized multi-apartment building is considerable.How people perceive the indoor climate is dependantnot only on the actual indoor temperature itself but alsoon other factors like air quality, individual metabolismand behaviour, radiation temperature and airmovement. In relation to this it can be noted thatprevious work have shown that about five percent ofany group of people will always be unsatisfied by theindoor climate [9], and that it is not possible to create aperfect climate that will make everyone happy.Figure 8: Outdoor temperature sensors placed in theshade (black line) and in full view of the sun (grey line)DISCUSSIONWhen dealing with temporary heat load reductions it isimportant to include the whole process of the reduction.This also includes what happens after the actual heatload reduction has been performed. For example, whenjust restoring the wanted control level after a longreduction, e.g night time set-back, the forward flowtemperature in the radiator system will rise much fasterthan the return flow temperature. This causes asubstantial, although temporary, heat load increase inthe radiator system which negates large portions of theenergy saving done during the actual reduction. Apartfrom decreasing the local net energy saving thisbehaviour is also less than desired from a system wideperspective, since it causes massive heat load peaks ifdone in many buildings simultaneously, e.g.contributing to morning peak loads. In order to avoidthis it is important to factor in the whole process of thereduction, and make sure that the control systemproperly handles the transition from the reduction levelto the original level. The inability among mostcommercially available control systems to properlyhandle this over-compensation is most likelycontributing a great deal to the lingering controversywhether night time set-back actually gives an energysaving or not.It is important to realize that the definition of anacceptable indoor temperature is not about having theindoor temperature at a certain precise level at all time,but rather to have it within a certain, sociallyacceptable, temperature interval at all time. This hasbeen discussed at great length in previous work [6].The general idea is that a greater temperature interval248CONCLUSIONSThere is an ongoing debate whether night time setbackslead to an energy reduction or not. Results fromthis study clearly show an energy saving in relation toheat load reductions, although this assumes that thecontrol system is able to smoothly handle the transitionfrom reduction to normal operation. The resultsshowing energy saving is evaluated in relation to thetotal energy usage which also includes tap-waterusage. Normally this is estimated to about 30% of thetotal energy use in a multi-apartment building.In prior studies of temporary heat load reductions thefocus has been on the fluctuations in the indoortemperature as a way of evaluating the energy saving[3]. This idea is based on the widespread notion thatany energy saving is linearly proportional to thetemperature difference between the indoor and outdoortemperature. This model might be true in a steady statesimulation where the temperature difference isassumed to have had time to permeate the air mass aswell as the entire building structure, but it is obviouslyinadequate in a dynamic situation. We have insteadfocused on the heat load and energy usage directly, i.e.the difference between forward and return temperaturein relation to the flow within the radiator circuit. In mostof the buildings evaluated there has been aconsiderable reduction of energy consumption withoutany noticeable change in indoor temperature. Thereason that there does not need to be a measurablechange of the indoor temperature is due to thedynamics of the thermal inertia of the building, e.g. thetime constant of a building is not a constant [6]. Thisaspect comes into play when using very short heat loadreductions, at most one or a few hours long. During thisfirst part of the reduction it is mainly the actual air massthat is influencing the indoor temperature drop sincethis body has a low resistance to change, i.e. the shorttime constant [10]. If the heat load reduction isprolonged, like during a night time set-back, the