Heat Gain and Loss Carried Out in Auxiliary Heating
The instantaneous thermal balance for a heated space can be given as follows:
If the right hand side of Eq. (1) is negative, there is no demand for heating; if this tendency continues, the indoor temperature will rise, leading to overheating. The increased indoor temperature will result in increased heat loss and accumulation, until equilibrium is reached. Ideally, the capacity of the building to accumulate the extra energy will be large enough to avoid overheating due to increased heat gains. Heat accumulated in the building is given to the space during the thermostat set-back period. This ‘loss’ of accumulated heat will be compensated when the heating system is on again and/or when there is solar heat gain. If the terms of Eq. (1) are integrated over a large-time interval, in this case a month, the accumulation term, qACC, becomes negligible; it is a more or less diurnal fluctuation of positive and negative values. Therefore, the monthly heat balance can be written as shown in Eq. (2), where QL,real is the integration of instantaneous losses qL over all hours, including hours when the instantaneous heat loss (qL) is increased due to overheating. The auxiliary-heating demand reduces to zero if the heat gain becomes larger than or equal to the heat loss. The real heat loss then is higher than the heat loss related to the set-point temperature; this means no portion of the heat gain has been utilized to reduce the heating energy demand, but has actually resulted in an increased internal temperature above the set-point temperature, which in turn resulted in an increased heat loss.
(2)
If the term QL, which is the integrated heat loss over a period of a month assuming that no overheating takes place, i.e. in cases of low heat gain or effective accumulation, is used instead of QL,real, the auxiliary-heating requirement can be expressed as shown in Eq. (3), where ?G, the utilization factor, is defined as the ratio of the utilized to the total heat-gains:
(3)
In the work described here, the definition of the utilization factor used previously has been adopted; however, it is applied only to the solar heat gain as shown in Eq. (4). Where there is internal heat gain, it is argued that the effect of this is to reduce the auxiliary heat gain by the equivalent amount.
(4)
where Qs is the sum of the solar, and Qi is the sum of the metabolic and incidental heat-gains from equipment. As the heat gain (Qs) increases, the auxiliary heating requirement approaches zero. For low values of Qs/QL, all the heat gains will be utilized in the space to offset part of the heat loss that would have otherwise been met by the auxiliary heating system. Therefore, for low values of Qs/QL, the utilization factor will be equal to unity.
In previous publications, the calculation of heat loss was carried out for a period of 4 weeks using monthly averaged weather data and an average daily value for ventilation. For large differences in the value of ventilation over the period of a day, the calculation was carried out for each part of the day reflecting the appropriate internal and external temperatures. To take account of thermostat setback during the non-working hours of a particular day, a correction factor (which is a function of the thermal losses and the thermal capacity of a zone) was applied. A correction for cold surfaces was also applied. The internal temperature was further adjusted taking account of working and non-working hours. The internal and solar gains for each month for a 10 h period were estimated taking account of working days and the mean internal heat-gain. The solar heat gain was also calculated by first calculating a shading factor and a coefficient to take account of the number of hours shading is applied. The calculation of heat loss and heat gain was based on a steady-state basis, with factors used to take account of dynamic heat flow. The auxiliary-heating requirement, however, was carried out using a detailed hour-by-hour thermal simulation program, DYWON. Finally, the utilization factor was calculated using Eq. (3).
While the methodology of determining the utilization factor in van Dijk and Arkesteijn’s was correct, the steady-state approach to calculating heat loss and heat gain means that only an average daily and monthly calculation is carried out leaving out the actual dynamic heat flow throughout the day. Therefore, the calculation of heat gains and heat losses does not take into account the complex nature of instantaneous heat flows. The parameters were determined on the assumption that buildings behave as single zones, which has been shown, in this work, to be inappropriate. There is a further and most important weakness in van Dijk and Arkesteijn’s work: the work was carried on the basis of three thermal mass categories, light, heavy and very heavy. The definition of these categories is not clear, thus allowing a grey area in the determination of heating energy between these categories as shown in Fig. 7. The following restrictive assumptions have also been made: no mechanical cooling; convective heating; thermostat controlled by air temperature; threshold for solar shading is 300 Wm?2; and no increased ventilation in the case of overheating.
The study reported here was carried out using a generic office building, divided into 14 zones, with east-facing and west-facing windows located at a latitude of 52°. The construction is steel frame; concrete floors; brick cladding; and pitched roof with concrete interlocking tiles on timber rafters, steel trussed to roof. Walls, ceilings, floors and windows are assumed to have average reflectances. Insulation is to the requirements of the Northern Ireland Building Regulations, and windows are double-glazed throughout. Glazing areas to the offices comprise 42% of the east and west elevations and permit good daylight penetration. It is assumed that the 16 m wide office is naturally-ventilated. Heating is provided by a low-temperature hot-water heating system. Ceiling-mounted fluorescent lights are used throughout, giving a lighting level of 300 lux. The weather data used in the study were for Kew, London.
Heat loss, auxiliary-heating energy requirement, internal heat gain and solar heat-gain are determined for each zone of the building for every hour of the month and year. The conditions for which the simulations were carried out were as follows: infiltration, one air change per hour; internal (incidental) heat gain set to zero; heat gain from (electric) lighting set to zero; heating set-point equal to 21°C; U-value of glazing, 3.3 Wm?2 K?1; heat loss to ambient environment for the whole building equals 5918 WK?1; and heat loss to the ground equals 196 WK?1.
To take account of the fact that the utilization factor is a function of weather and the building’s thermal-response, the determination of correlation parameters k and D was carried out using SERI-RES, a dynamic thermal simulation tool where detailed hour-by-hour calculations can be carried out using hour-by-hour weather data. Heat loss, auxiliary heating energy requirement and solar heat gain were determined for each zone of the building. In SERI-RES, all interrelated factors, i.e. thermal mass, prevailing zone temperature (which may deviate from the thermostat set-point), internal heat gain from people, lighting heat gain and ventilation are considered on an hour-by-hour basis simultaneously.
Tags: heat gain
