WO2007061357A1

WO2007061357A1 - Method of controlling the heating of a building at a desired level

Info

Publication number: WO2007061357A1
Application number: PCT/SE2006/001309
Authority: WO
Inventors: Roger Taesler
Original assignee: SVERIGES METEOROLOGISKA OCH HYDROLOGISKA INSTITUT
Current assignee: SVERIGES METEOROLOGISKA OCH HYDROLOGISKA INSTITUT
Priority date: 2005-11-22
Filing date: 2006-11-17
Publication date: 2007-05-31
Anticipated expiration: 2008-05-22
Also published as: SE529210C2; SE0502551L

Abstract

The present invention relates to a new method of controlling the heating of a building at a desired level. Firstly, building specific characteristics in the form of technical building characteristics and data on mode of operation of the building and building location are established, and meteorological parameters for the location of the building are forecast. Based on this a forecast equivalent temperature for the building is then calculated for chosen future points of time, such that there is a linear relation between heating power supply and the difference between the desired indoor temperature and said equivalent temperature. Finally, heat is supplied continuously in dependence of said difference.

Description

Method of controlling the heating of a building at a desired level

The present invention relates to a new method of controlling the heating of a building at a desired level.

Heating of buildings accounts for a very large proportion of the end use of energy in Sweden and emissions therewith associated affecting the environment. Thus, methods for heating control giving an even indoor air temperature with a minimum use of externally supplied energy are of great economic and environmental importance.

Automatic systems for control of heating of buildings in response to either indoor or outdoor prevailing air temperature are known. Normally the outdoor air temperature is used as a reference. Control in response to the indoor air temperature has the disadvantage of always lagging behind the impacts of weather.

Traditional control in response to outdoor air temperature does not take into account the impacts of other weather factors, e.g. solar radiation, wind etc., on the building heat requirement. Experience also shows that the heat requirement is not a simple, linear function of the outdoor air temperature, cf. figure 1. Heat supply control curves based on outdoor air temperature therefore usually have one or several breakpoints which need to be repeatedly readjusted in accordance to experience for each building, cf. figure 2. In addition, the use of different control curves during different seasons may be necessary.

A number of factors other than the outdoor air temperature are previously known to influence the building heat budget. The demand of energy for heating of a building depends on its construction and mode of operation in combination with local weather conditions. The most important weather parameter is the temperature of outdoor air but in addition i.a. wind and solar radiation significantly influence losses and gains of energy. Cloudy and windy weather conditions increase the heat loss whereas solar radiation falling on the building reduces the heat loss. Furthermore, internal heat gains from occupants and use of electricity contribute significantly in some buildings.

If the sum of all heat losses exceeds the "free gains" from solar radiation and internal heat sources, there is a net deficit of heat which must be compensated by external energy ("purchased energy"). The task of the control system is to maintain a desired indoor temperature with a minimum supply of external energy.

The "free energies", i.e. the sum of passive solar gain and internal heat often cause a temporary surplus in the building heat balance even during the heating season, in particular in modern, well insulated buildings with large window areas. With traditional control this results in excessive indoor air temperature or a need for increased ventilation or even cooling. If these free energies could instead be made available for heating a significant proportion of the total building heat requirement during the heating season could be covered. It is an advantage in controlling the heat supply to take into account also the thermal "inertia" of the building (i.e. the building mass and energy technical qualities such as insulation, air tightness etc.) that retards and dampens the impact of a change in the building heat balance on the indoor air temperature. One way of achieving this is to reduce the supply of heat in advance of a future surplus or vice versa regarding a deficit.

The basic function of the present invention is to make useful the "free energies" when controlling the heating of buildings. This requires a forecast control method based on a calculation of the building heat balance. A further developed version of the invention also takes into account the thermal inertia of the building.

The present invention offers a new solution of the problem of maintaining the building indoor air temperature by adjusting the heat supply in response to a so- called "Equivalent Temperature" (ET) that is obtained from the calculated require- ment of net heat from external sources. A linear relation between the net heating requirement and the ET expresses the effect of weather on the ET.

The invention solves the stated problem by being designed as is evident from the following independent claim. The remaining claims define advantageous embodi- ments of the invention.

In the following the invention will be described in more detail with reference to the accompanying drawings, in which Fig. 1 is based on operational data from a major municipal district heating plant and shows the daily sum of net heat load as a function of daily mean outdoor air temperature,

Fig. 2 shows examples of supply water temperature curves when controlling by the outdoor air temperature and according to this invention by the equivalent temperature, respectively, and

Fig. 3 shows the heat budget of a building.

Figure 3 shows the dependence of the heating net requirement N on several different components. N > 0 corresponds to a heat deficit (heating requirement) and Λ/ < 0 corresponds to a heat excess. N can be expressed as

N = C + I + F + G - EP - S (1)

where

C is heat loss due to conduction through the building envelope (roof, walls and windows),

/ is heat loss due to natural ventilation (infiltration and exfiltration) F is heat loss due to mechanical ventilation

G is heat loss to the ground

EP is heat gain from electric appliances and occupants

S is heat gain from solar radiation through windows

Naturally, in calculations for a specific case one or more of the terms in can be set to zero, if negligible in comparison with remaining terms.

The invention is based on calculation and subsequent summation of the individual terms in Eq. (1) to obtain the total net requirement for supply or loss of heat in order to maintain a constant indoor air temperature. This net requirement is then expressed in terms of an equivalent temperature, ET, that can be used to replace directly the prevailing outdoor air temperature as input to the building automatic heating control system, also in already installed control systems. By definition the supply water temperature is a linear function of the difference between ET and the desired indoor air temperature, cf. figure 2, which is a strong advantage in the control process. The starting point for calculations of ET is a meteorological forecast on air temperature, wind and solar radiation. The latter parameter is calculated as a function of cloudiness, visibility, air humidity and precipitation. For someone who, unlike a man skilled in the art, is unfamiliar with this calculation, Taesler, R. and Andersson, C, 1984: A method for solar radiation computations using routine meteorological observations, Energy and Buildings, Vol. 7 pp 341 - 352 is hereby incorporated by reference. Preferably, the meteorological forecasts have a time resolution of 1 hour. Naturally, other time resolutions may also be used.

Applications of the invention involve taking into account basic technical data on the building construction and its mode of operation as well as the influence of the location of the building site and its surroundings on the meteorological input data. This may involve a completely individual calculation for each single building. In many cases it is possible, however, to define building characteristics by reference to different typical cases. For example, all buildings of a certain age and general type of design may be assumed to exhibit certain values i.a. on thermal conductivity through exterior walls, roofs and windows. This applies also to other aspects of the building.

In calculating forecast values on ET one or several data items under each of the following headings are used:

Technical building characteristics • Data on the geometrical dimensions of the building (length, width and height)

• Data on heat insulation (U-values) for roofs, walls, floors and windows

• Data on air tightness of the building (design air exchange rates)

• Data on mechanical ventilation rates including degree of exhaust air heat recovery • Data on the proportion of window glass area of each building facade

• Data on the main orientation of the building

• Data on window solar radiation transmittance (transmission factor)

• Data on building heat storage capacity (time constant) Mode of operation of the building

• Data on available internal heat during different parts of the day for different weekdays. This concerns heat emitted by electric or other appliances as well as from occupants. It is possible to account individually for this supply for a build- ing, but quite often data are used for different representative building cases - dwellings, schools, offices etc.

• Data on desired indoor air temperature.

Data on technical building characteristics and mode of operation are obtained by means of filling out an input data form.

Building site location and surroundings

• Data on the geographical coordinates of the building (forecast point)

• Data on the type of built up area and "ground roughness" of surfaces surround- ing the forecast point (surface roughness coefficient for wind calculations).

• Data on reduction of the local horizon (due to other buildings, heights etc.)

Data on reduction of the horizon due to surrounding built up areas, topography etc. are obtained through the input data form mentioned above. Remaining data are determined as part of the meteorological production process.

Meteorological parameters

• Forecast data on outdoor air temperature

• Forecast data on wind direction and wind velocity • Forecast data on cloudiness, visibility, air humidity and occurrence of precipitation.

Meteorological parameters included in the model can be obtained from normally available meteorological forecasts in a known manner. To improve the accuracy it is recommended to apply corrections to the meteorological parameters by accounting for influences due to the local surroundings of the building - type of buildings, surface "roughness" etc. How this is to be done is well known to a man skilled in the art and for a man not skilled in the art guidance is given in the literature referred to in the following detailed description of the model. In the following detailed description of the model for the equivalent temperature the net heat requirement N is expressed by Eq. (1). The heating requirement is taken, as already mentioned, as being proportional to the difference between the desired indoor air temperature and the equivalent temperature ET, i.e.

from which it follows that

ET = T₁ - NZk (3)

where N stands for the instantaneous net heat requirement for maintaining indoor air temperature T₁ and k is specific for each defined case of building type. The heating demand has the dimension Watt per m² (W/m²) of heated floor area. Eq:s (2) and (3) imply that a single straight line with slope k can be used as control curve for the supply water temperature in a central heating system.

Methods for computation of each of the components in Eq. (1) are well known to a man skilled in the art. The concrete calculation of the different parameters can be performed by various known methods and does thus not constitute any part of the present basic invention. For those who, unlike a man skilled in the art, are not familiar with these calculations a brief presentation of a typical calculation is given in the following.

The different components in Eq. (1) are calculated for a single-cell building, i.e. without taking into account internally separating walls or floors. The indoor air temperature is taken to be uniformly constant in the entire building, usually = +21 ⁰C. All components are calculated as hourly values using observed or forecast meteorological data.

The heat transmission through walls and roofs (component C) is calculated as

C = u_e Δ T,

where AT = (T₁ - T₀) is the difference between indoor and outdoor air temperature and u_eis an effective area-weighted mean value of the heat transfer coefficients of the entire climate shell (walls, windows, roofs) above ground. As input data for calculation of the U-value, the Building Code or equivalent standard valid for the year of construction of the building can be used. Alternatively, data valid for the individual building are used. Data and calculation methods for this are found e.g. in Hagentoft, C-E., 2001 : Introduction to building physics, Studentlitteratur, Lund, which is hereby incorporated by reference. See also ASHRAE Handbook 1981 Fundamentals, American Association of Heating, Refrigerating and Air-Conditioning Engineers Inc., Atlanta, also hereby incorporated by reference.

Heat losses due to mechanical ventilation (component F) are also proportional to the indoor - outdoor temperature difference, AT = (T, - T₀), and are calculated as

F = nVp C_p AT,

where n stands for the number of air exchanges per hour, V is the heated and ventilated volume, p is the density of indoor air and c_p is the specific heat capacity of the air. Differences in ventilation rates by day and by night are taken into account. The losses are reduced by taking into consideration a known or estimated degree (%) of heat recovery from the exhaust air.

The heat loss due to natural ventilation (component I) is calculated as

l = f(ΔP) ΔT Vp c_p ,

where f(ΔP) is the number of air exchanges per hour as a function of the indoor - outdoor pressure difference (AP ) across the climate shell. The leakage factors necessary are computed from the design leakage value valid according to the Building Standard for the year of construction or a corresponding document. The pressure difference depends on the indoor - outdoor temperature difference (AT) and the wind velocity at roof ridge level as transformed by calculation from the velocity at the reference level 10 m above ground and taking into account also the exposure of the building to wind (the influence of surrounding terrain and buildings as well as the orientation of the building relative to the direction of the wind). Theory and methods useful for the transformation to roof ridge level are found e.g. in Cook, N.J., 1985: The designers guide to wind loading of building structures, part 1, Building Research Establishment (BRE) Garston, Butterworths, London and in Plate, E.J., 1971 : Aerodynamic characteristics of atmospheric boundary layers, AEC critical review series, National technical information service, US Dep. of Commerce, Springfield, Virginia, which are hereby incorporated by reference.

The wind pressure on the building is calculated using shape factors (pressure coefficients) for each partial surface of the climate shell. More information may be found in AIVC, 1984: Wind pressure data requirements for air infiltration calculations, Technical Note AIC 13, Air Infiltration Centre, Bracknell or in Wiren, B., 1985: Effects of surrounding buildings on wind pressure distributions and ventilation losses for single- family houses. Part 1: 11/2-storey detached houses, M 85: 19, Statens institut for byggnadsforskning, Gavle, which are hereby incorporated by reference.

The computation of the function f(ΔP) is described in detail in Taesler, R., 1986: Climate, buildings and energy exchange - an integrated approach. Tekniska meddelanden nr 297, Inst, for uppvarmnings- och ventilationsteknik, KTH,

Stockholm, which is hereby incorporated by reference. As regards models and methods for calculations of natural ventilation cf. e.g. ASHRAE Handbook 1981 Fundamentals, American Association of Heating, Refrigerating and Air-Conditioning Engineers Inc., Atlanta or Herrlin, M. K., 1992: Airflow studies in multi-zone build- ings. Models and applications, Dep. of building services engineering KTH, Swedish Council for Building Research, Stockholm, both incorporated by reference herein.

Heat losses to the ground (component G) are small and may be approximated as constant and proportional to the indoor temperature (T₁) and annual mean outdoor temperature (T₀ ), ΔT_yr = (T₁ - T₀ ).

G = k_G ΔT_yr ,

where k_G is the heat transmission coefficient of building foundation.

Heat gains from use of electric appliances and occupants (component EP) depend on the type of use and mode of operation of the building. This heat gain can be calculated or estimated. Estimates may be based on Boverket, 1994: Byggnaders varmeenergibehov, utgangspunkter for omfόrdelningsberakning, Handbok, Boverket, byggnadsavdelningen (The National Board of Housing and Planning, Division of Building Construction), Karlskrona, which is hereby incorporated by reference. The passive solar heat gain (component S) is computed from calculated direct and diffuse solar irradiation on each building facade taking into account the orientation of the building and reduction of the free horizon due to surrounding objects. The computation is described in detail in Taesler, R. and Andersson, C, 1984: A method for solar radiation computations using routine meteorological observations. Energy and Buildings, Vol. 7 pp 341 -352, which is hereby incorporated by reference. The solar gain is obtained after reducing the calculated irradiation on window surfaces with regard to the transmissivity of the window glass (2 or 3 window panes) and permanent solar screening devices (Venetian blinds etc.) if such are installed. The necessary data on glass transmissivity may be found e.g. in Brown, G. and Isfalt, E., 1974: Solinstralning och solavskarrmning, Rapport R 19:1974, Statens Rad for Byggnadsforskning (The National Swedish Counccil for Building Research), Stockholm, which is hereby incorporated by reference.

The above leads to the following equation for the net heat requirement N

N = (A+Bχv²)χΔT + G - EP - S (4)

The coefficients A and S are determined by multiple regression on Eq. (4) with weather data over an extended period, e.g. one full year of hourly weather data. Each individual building and each case of building type is characterized by values on the regression coefficients A and β. Thus, one particular combination of meteorological data (7^", v, S) results in different values on ET depending on the characteris- tics and position (exposure) of the building.

From Eq:s (2) and (3) the following expression for the equivalent temperature ET is now obtained.

ET = T- (1/A) X(BxV²XAT + G -EP - S) (5)

Eq. (5) implies that ET can be either higher or lower than the outdoor air temperature depending on if the "free energies" (the heat gains EP + S) exceed or fall below the heat losses due to wind and temperature. A net heat requirement is present when ET < T₁. In order to utilize the heat storage capacity (the thermal inertia) of the building the heat supply must be adjusted ahead of time. The building thermal inertia is usually assessed by its time constant. For someone who, unlike a man skilled in the art, is unfamiliar with the calculation of this time constant, which is done based on technical data for the building, is directed to Petersson, B-A., 2001 : Tillampad byggnadsfysik, Studentlitteratur, Lund, which is hereby incorporated by reference.

The thermal inertia can be accounted for partly by smoothing the forecast equivalent temperature using weighting coefficients calculated in account of the building ther- mal constant and partly by introducing a shift in time of the course of the smoothed equivalent temperature. Usually, the time shift is 1 - 3 hours.

The theory for accounting for the building thermal inertia is briefly described below.

Taking into account depletion or accumulation of heat in the building mass the external supply of heat required at an instant in time t for maintaining the desired indoor temperature is

N^'(t) = N(t) + ΔN(t) (6)

The corresponding smoothed equivalent temperature is

ET^'(t) = ET(t) + ΔET(t) (7)

An expression for ΔET(t) can be derived as follows. An exponential increase or decrease ΔN(t) of heat storage in the building over time interval £ results in a corresponding change in the smoothed equivalent temperature by

ΔET^'(t) = ΔET(t) e^~t/τ (I - e^~1/r ) (8)

where r is the building time constant.

Then, the smoothed equivalent temperature at time t₀ is

tc-δ ET(t₀) = ET (I₀) + ∑ΔE7'(f), t=-τ i.e. t_a-s

ET(to) = ET (to) + ∑ AET(ty^/T (1 - e-^1/r ) (9) t=-τ

Eq. (9) can be written as

fo-<y

EF(f ₀ ) = E7(f ₀ ) - Y ET % ) + X a(f )E7^"(f „ - 1) (10) t=-τ

where r = ∑a(f) ^and a(t) = e^~t/T (ϊ - e^~Vτ ) (11) f=-r

The coefficient γ and the time constant r are related by lim_r→∞ γ = 1 - 1/e=0,632. Corresponding values on γ and r are shown in the following table.

Values used are chosen in the interval 12 < τ< 24 hours.

Eq. (9) has the effect of causing a momentary unbalanced heat budget. During time intervals of falling ET^' the heat supply will be insufficient to compensate for the total loss and vice versa during periods of rising ET^'. The smoothing reduces variations in heating power, in particular peaks, but tends on the other side to cause a deviation in the indoor temperature from the desired, constant level. This tendency is counteracted by a decrease or increase of the heat stored in the building. To keep the indoor temperature within acceptable limits, values chosen for the time constant in Eq. (9) must not be too large.

To control the effect of heat storage on the diurnal variation in indoor temperature the equivalent temperature ET' according to Eq. (9) can be further modified as follows. ET ^'(to) = ET(to)+ AET(to)e<-^m (12)

where

AET(t₀) = E_e(t₀ + <*) - ET(t₀) (13)

The time step δt causes a forward phase shift. This does not affect the daily energy balance.

It goes without saying that the method according to the invention can be applied in new heating control systems, but also in already installed systems. In both cases the outdoor air temperature is replaced by the equivalent temperature for the control procedure.

In practise values of the equivalent temperature, being based on, inter alia, meteorological forecasts, are computed by someone competent of making these, e.g. the SMHI, and distributed in the form of data files to subscribers for use in the control system of their buildings. For practical reasons forecast data are distributed for periods of a number of days ahead, usually 5 days, but other periods are of cause possible.

Claims

Claims:

1. A method of controlling the heating of a building at a desired level, c h a r a ct e r i s e d in that building specific characteristics in the form of technical building characteristics and data on mode of operation of the building and building location are established, meteorological parameters for the location of the building are forecast, a forecast equivalent temperature for the building is calculated, based on the building specific characteristics and the meteorological parameters, for chosen future points of time, such that there is a linear relation between heating power supply and the difference between the desired indoor temperature and said equivalent temperature, and heat is supplied in dependence of said difference between the desired indoor temperature and said equivalent temperature.

2. A method as claimed in claim 1, c h a ra ct e r i s e d in that the meteorological parameters are corrected in view of the effect of the local surroundings of the building, and the so corrected meteorological parameters are used in the calculation of the forecast equivalent temperature.

3. A method as claimed in claim 1 or 2, c h a r a ct e r i s e d in that the thermal inertia of the building is taken into account by smoothing the equivalent temperature in account of the building time constant.

4. A method as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the thermal inertia of the building is taken into account by introducing a shift in time of the course of the equivalent temperature.

5. A method as claimed in any one of claims 1-4, c h a r a c t e r i s e d in that the technical building characteristics comprise one or more of the following items, data on the geometrical dimensions of the building, data on heat insulation for roofs, walls, floors and windows, data on air tightness of the building, data on the degree of exhaust air heat recovery, data on the proportion of window glass area of each building facade, data on the main orientation of the building, data on window trans- mission factor, and data on building heat storage capacity.

6. A method as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that information about the mode of operation of the building comprise one or more of the following items, data on available internal heat emitted by occupants and heat emitting appliances during different parts of the day for different weekdays, and data on desired indoor air temperature.

7. A method as claimed in any one of claims 1-6, c h a r a c t e r i s e d in that the meteorological parameters comprise one or more of the following items, forecast data on outdoor air temperature, forecast data on wind direction and wind velocity and forecast data on cloudiness, visibility, air humidity and occurrence of precipitation.

8. A method as claimed in any one of claims 2-7, c h a r a c t e r i s e d in that the information about the building site local surroundings comprise one or more of the following items, data on the type of built up area, data on the ground roughness for local wind calculations, and data on reduction of the horizon around the building due to buildings and heights.