Modelling of Modern Dwelling Building in Bulgaria

The impetuses for the Building Energy and Environmental Simulation are the growing activities in improving energy efficiency of building performance, quality of life and sensitive to the interconnectivity with the ecology of planet Earth. Until recently, fossil fuels were considered essential to industrial growth and living standards. Air and water pollution, and adverse effects of a climate and the environment were of secondary importance.

Analytical models are the tools in which both development and financial decisions are made. At present, most energy-making is based on old financial paradigms focused on the weighted costs of capital. New approaches to define the risks of investment and the cost associated with these risks must be more adroitly incorporated into financially decision-making. Risks associated with energy choices transcend simplistic environmental externalities to security of supply, waste disposal, transport costs and access to supply, labour and capital.

It is in this context that new and accurate methodology for energy calculations is most crucial for the design and analysis of the performance of space heating and cooling systems. It is generally accepted that buildings can be designed to be energy effective if their thermal insulation is increased; window size, air leakage, and lighting level decreased; shading devices properly installed; heating and cooling system adequately designed, installed, and maintained; and their storage capability most fully utilized. These energy saving features, however, must be considered with reference to numerous constraints, such as added costs for material, construction and maintenance, conformance to local building codes, occupancy life styles, aesthetics, construction practices, and availability of equipment.

• Simulation model of Building
• Energy Efficiency
• Thermal Comfort
• Daylight
• Conclusions

Simulation model of Building includes the following parts: valid data model (description of the building, climatic data for the concerning region), simulation s procedures and computer programs for simulation processing, analyze of received results etc. To calculate the heating and cooling requirements for an entire building generally will require extensive thermal analysis.

The following computer programs we have used (created by ECOTHRTMINGEENERING Ltd and West-Sout University in Blagoevgrad) to make energy simulations of buildings:

A computer model named HC_LOAD is used to collect and analyze building construction data, climatic data, and data for external and internal gains to the building.

A computer model named SES_LOAD calculates seasonally and yearly energy requirements of the buildings. This program uses data from computer model HC_LOAD (maximal value of a heating or cooling load) and monthly or annual temperature distribution (table 2, fig.3) to calculate monthly, seasonally or yearly energy for heating or cooling.

A computer program SOLAR was created to calculate direct solar gains to the living space of building through the windows. This program uses available climatic data for a solar radiation and optical characteristic of the transparent elements of the used windows in the building.

In any cases we use program PASS_SYS to simulate thermal performance of indirect passive solar systems with a massive wall. We use this program also, to investigate the transient thermal processes in massive building constructions when that is needed. This program solves the transient equation of thermal conductivity in walls taking into account all thermal processes on the boundary (convection, radiative thermal exchange, solar radiation etc.).

A computer model named NET_SYS was created to make a detailed calculation for a heating and cooling installations. This includes: hydraulic calculations for pipe nets, sizing a heating and cooling devices, heat exchangers, pumps, controlling and regulating elements etc.

General Information

The first step of Building Energy and Environmental Simulation is the description of the building. This includes: schemes or photo of the building, dimensions and composition, orientation, buildings' function, urban structure of the site and dwelling complex etc.

Worked example

The dwelling type chosen for a presented example is the modern building of a new dwelling complex situated near Bulgaria buld. in south of Sofia . This is a block numbering No 40-42. The building is oriented on a north-south axis (see figure 1a) and has well glazed west and east facades. The Eastern and Western facade of the building are presented on fig.ures1 and 2. Northern and Southern sides of building are attached to other buildings. Figure 3 shows the vertical cross-sectional configuration of the building. Several buildings form detached group comprising three and four story blocks with inner yard and garden.

Fig. 1. Eastern facade of the building

Fig. 2. Western facade of the building

Fig. 3 Vertical cross-section

The building is four story block with two separated entrances. The building will be equipped with central heating system using liquid fossil fuel

To improve design and thermal performance of the building, the ordinary masonry walls and double wooden windows were foreseen.

Natural ventilation is possible by opening the western front doors and all the eastern windows to get across flow of air through the building.

The base characteristics of the building are received by computer module HC_LOAD:

Total volume of the building Vt - 3460 m3
Total surface area Ft - 1792 m2
Total external doors and windows� area - 263 m2
Building construction parameter Ft/Vt - 0.594

Two sets of climatic data are needed to conduct analysis of energy performance of buildings. The first is the set of extreme climatic conditions that determine the installed energy capacity of plant components of the building. This includes: extreme temperatures, wind velocity and wind direction, humidity, maximal solar radiation etc. These are required to calculate maximal heat losses of a building and heat inputs to the building space in summer from the environment.

The second sets of climatic data are required to assess energy needed for heating and cooling of building during the year. For such kind of analysis it is best to use full years� data, or full seasonal data if the process is a seasonal one, and if data are available for many years it is necessary to select the best set.

The set of full climatic data for yhe year is termed as a reference year which the best data for energy analysis of buildings, and is a result of many years� collection data. For Bulgaria such a data is available only for Sofia and partly for other regions in Bulgaria.

The base geographical and climatic data for Sofia that are needed for the building simulations are :


Location : Sofia
Latitude : 42o 45' North
Longitude: 23o20' East
Elevation: 550 m

Climate :
Average Annual Temperature :
Winter minimal temperature ( 0 hour�s annual unassurance) : -27oC
Winter minimal temperature ( 20 hours annual unassurance) : -16oC
Winter temperature (400 hours annual unassurance): - 5oC
Summer maximal temperature (0 hours annual unanssurance) 37oC
Summer maximal temperature (20 hours annual unanssurance) 34oC
Summer temperature (200 hours annual unanssurance) 27oC

Average annual wind speed 3.7 m/s
Preliminary wind direction west
Heating degree-days (troom=19oC) 2900
The winter design temperature (for heating load calculation) is -16oC, and summer design temperature (for cooling load calculation) is 34oC.

Table 1. Monthly ambient temperatures for Sofia Region

Month	1	2	3	4	5	6	7	8	9	10	11	12
Tav	-2.4	-0.1	3.9	10.0	14.6	18.0	20.2	20.2	16.5	11.0	5.2	0.1
Tmax	0.9	4.1	9.0	15.4	20.5	23.7	26.6	26.6	23.0	16.6	10.1	3.6
Tmin	-6.3	-4.8	-1.3	3.9	8.8	11.9	13.8	13.5	10.3	5.7	2.3	-3.4

Table 2. Monthly and annual frequency distribution of ambient temperature for Sofia [hours]

Table 3. Daily solar radiation on a horizontal surface [W/m2] for representative day (monthly)

Walls and windows of the building provide the key part of energy transmission between building space and environment. Windows are the weakest elements in the overall thermal insulation of building, but also tend to be a key path to direct gain solar radiation for heating. Thermal insulation of walls is an important construction characteristic in energy performance of heating and cooling processes.

It is generally agreed that for domestic activities ventilation rates between 0.5 and 1 ac/h are sufficient

Of primary importance to the building and the owner will be the amount of insulation utilized during construction, as well as careful attention to the passive solar considerations noted above. In general, in this days of increasing fuel costs, having extra insulation in the walls and selling of a building will pay for themselves over a few years time.

Worked example

The characteristics of the walls and windows foreseen in initial variant of building design was:

 
external walls - 250 mm masonry         - R = 0.680 m2oK/W
ceilings                                - R = 1.300 m2oK/W
floors                                  - R = 0.760 m2oK/W
wooden windows  -                       - R = 0.324 m2oK/W 


Infiltration coefficient 0.43 m3/(h.m.Pa2/3)

The overall characteristics of the building are received by computer module HC_LOAD :

Overall U-value for windows and doors:          3.103 W/m2oK
Overall U-value for walls                       1.811 W/m2oK
Overall U-value for building                    1.366 W/m2oK

Heating and cooling systems 

Heating and cooling systems provide needed energy for cover the heat losses in winter or compensate heat inputs in summer. There are different types of building plants that cover the requirements for energy.

In the working example, we analyse passively collected solar energy by transparent elements of the building on the western and eastern facade.

The main heating system for the working example is central heating (hot water boiler) which serves the group of nine blocks forming a separated group in the complex .

The heating system is ordinary water convective installation that uses liquid fossil fuel as an energy source. A thermal regulation in every room is suggested and measuring of consumed energy for every apartment.

The program NET_SYS was used to sizing heating devices, pipeline network and controlling elements of heating installation. An aluminum heaters, plastic pipeline network and thermal regulators for all rooms are the main properties of the heating installation for considered building.

Cooling installation for this building is not foreseen. Overheating will be controlled by blinds and curtains and regulating an internal gains.

Additional systems - lighting, electricity, people

Any other systems have contribution to the energy balance of the building. Lighting, electricity, cocking devices and people extract heat energy to the building space. This heat must be assessed in overall energy balance of the building.

Computer model

Computer model is a realization of simulation modeling of energy performance of buildings. It comprises a number of program modules addressing project management, simulation, result recovery and display, database management and report writing. The computer model usually allows the interactive definition of building and plant configuration, manages and analyses climatic and construction data collection, calculate the time dependent energy and fluid flows, make energy balance for different elements of buildings, generate tabulations, reports, graphical outputs etc.

Purpose of computer model

Computer model is a transient energy simulation system that is capable of modeling the energy and fluid flows within combined building and plant systems when constrained to conform to control action. It is applicable to existing buildings and new designs, with or without advanced technological features. The computer model is enabled the user to answer more design question

- What and when are the peak building or plant loads and what are the causal energy flows?

- What will be the effect of some design change, such as increasing wall insulation, altering the window shape and size, changing the glazing type or distribution, introducing daylight control devices, re-zoning the building, re-configuring the plant or changing the heating/ cooling control regime?

- What is the optimum plant start time or the most effective algorithm for weather anticipation?

- How will comfort levels vary throughout the building?

- What benefits can be expected from the different possible lighting control strategies?

- What are the relative merits of different heating and cooling systems and their associated controls?

- How will temperature stratification, in terms of zone sensor and terminal unit location, affect energy consumption and comfort control?

- What contribution does building infiltration and zone-coupled air flow makes to total boiler or chillier load and how can this be minimized?

- How do suggested design alterations affect air flow and fresh air distribution (i.e. indoor air quality) within the building?

- What is the effect of special glazing (such as thermotropic, holographic, low-e or electro chromic glazing) on summer overheating?

- Which are the benefits from architectural building features such as atria, sun spaces, courtyards, etc.?

- What is the contribution (in terms of energy saving and thermal comfort) of a range of passive solar (heating or cooling) features?

- What is the optimum arrangement of constructional elements to encourage a good load leveling and hence efficient plant operation?

- What are the energy consequences of a noncompliance with prescriptive energy regulations or conversely, how should a design be modified to come within some deemed-to-satisfy performance target?

- Which heat recovery system performs best under a range of typical operating conditions?

There are many computer programs which use different mathematical techniques for building simulations. It is difficult to find out model answers completely all questions mentioned above.

Computer modeling of performance

The mechanism of energy use in buildings is complex, involving three main factors; the physical building itself, the efficiency of the energy-using equipment such as heating plant and lighting, and the way that occupants control the building and system. Different combinations of these factors lead to a wide variance in energy use.

Calculation of the energy requirements of the heating and cooling systems of a building involves three major steps that may be carried out simply to achieve approximate results, or with increasing degrees of complexity and sophistication as more accurate and more refined determination of system performance is required. First, is the calculation of heat loss or heat gain to the space which is heated or cooled. Second, is the determination of the heating and cooling load imposed on the system. Third, is the calculation of the energy input to all of the system components to satisfy that load.

Customarily, heat load calculations are made for the so-called "design conditions" for sizing the equipment and developing the design of the heating and cooling system. However, the "design conditions" normally exist only for very few hours, if at all, during a heating or cooling season. Consequently, the actual day-by-day and hour-by-hour heating and cooling load for energy consumption is quite different from that for the design condition. Thus the heating and cooling load calculation for the purpose of estimating "energy requirements" must reflect the actual weather conditions rather than a design conditions.

Worked example

The program HC_LOAD give building's heating loads for the design conditions:

Heating load for the building           - 109594 W
        - heat conductivity             -  82300 W
        - infiltration                  -  27294 W
Average building temperature            -   19.4 oC
Specific heat load                      -   22.7 W/m3

Simulation

The most accurate calculations take into account the R-value of the floor, ceiling, walls, doors, windows, etc. It should be noted that one of the largest losses in heat is the heat loss by infiltration caused by air changes in the house. That is whenever the doors are opened, windows are open, flue damper in the chimney is left open, and so forth, heat escaped and cold air comes in. A sample case estimated that one air change was made every hour of the entire volume of air within the heated living space.

A cooling (heating) load is the amount of energy that is transferred to (from) the room and simultaneously removed (added) by the conditioning equipment at any given time of interest. To calculate this quantity directly requires a rather laborious solution of energy balance equations involving the room air, surrounding walls, infiltration and ventilation air, and internal energy sources.

Table 4. Daily distribution cooling load for a representative day of August

Typical room with a large glazing area on the eastern facade

Typical bedroom with glazing area on the west facade

The cooling load varies with time.as shown in Figure 4. The blue area is the Qbuild row data from table 4.

Energy for heating of building is calculated by computer model SES_LOAD. For the worked example was used data for ambient temperature distribution shown in table 1.

Table 5.

Results analysis

Results from energy efficiency calculations for the initial building design show that the main thermal properties of building do not response to the modern requirements of thermal performance of buildings. The overall U-value of 1.366 W/m2oK and seasonal energy of 227736 kWh are not acceptable for modern building. Because of existing requirements of energy efficiency of building, an improved building design was suggested. Corresponding analysis of this design is presented in next paragraphs.

Energy Efficiency

Introduction

For an existing house the owner should first consider good building use habits in reducing heating and cooling requirements. Second should be considered storm windows and doors, grouting and putting to close down cracks in the building and other good housekeeping practices. Third, insulation should be considered.

The actual heating and cooling requirements of course will depend to a certain extent on number of inhabitants of the building and their habits, the use of the washer and dryer, the lighting systems, etc.

It is easy to see that there are five primary ways to lowering the heat requirements for any building:

1. The first is to turn down the thermostat so that less heat is required in the first place. This also reduces the rate at which heat leaks out of the building. However, the limit to turning down the thermostat, of course, is the comfort of people inside.
2. The second factor is keeping the doors and windows shut.
3. The third factor is the tightness of fit of doors, windows, and all other openings to the inside.
4. The forth factor is double windows and storm doors. R-values for storm windows over a single pane flat glasses are roughly twice as large..
5. The fifth factor is the insulation in the walls, floors, and ceilings.

An effective way to design the heating and cooling systems which is the optimum from the standpoint of energy consumption, peak power demand and many practical constraints mentioned above, is to study the building thermal performance using accurate simulations. The use of computer simulations makes it possible to evaluate the sensitivity of various design alternatives on the net energy usage, they can be very effective tool in the design process.

Project - description

The results presented in paragraph 1.2 defined the overall thermal characteristics of the building. According to the existing requirements in Bulgaria for the energy efficiency of building an overall U-value for building (for the construction parameter Fo/Vo = 0.594) must be below 1.037 W/m2oC. Because of necessary to cover these requirements, a new walls and windows, was suggested.

The thermal characteristics of this building elements are:

 
External walls 
        - 250 mm masonry + 40 mm mineral wool           - R = 1.880 m2oK/W
ceilings                                                - R = 1.300 m2oK/W
floors                                                  - R = 0.760 m2oK/W
wooden windows                                          - R = 0.324 m2oK/W  
          infiltration coefficient 0.11 m3/(h.m.Pa2/3)

The overall characteristics of the building are received by computer module HC_LOAD:

 
Overall U-value for windows and doors:                  3.103 W/m2oK
Overall U-value for walls                               0.826 W/m2oK
Overall U-value for building                            1.118 W/m2oK

Heating load for the building           - 78448 W
        - heat conductivity             - 63890 W
        - infiltration                  - 14558 W
Specific heat load                      - 22.7 W/m3

This construction does not cover the existing requirements for the energy efficiency of building, also. This is because of the big windows� area, situated on external walls of building. Next option, we have considered in this project was to using night insulation of windows (blinds and curtains). In this case, the windows� R-value was increased to R=0.48 m2oK/W.

The overall characteristics of the building are:

 
Overall U-value for windows and doors:                  2.221 W/m2oK
Overall U-value for walls                               0.826 W/m2oK
Overall U-value for building                            1.005 W/m2oK

Heating load for the building           - 69780 W
        - heat conductivity             - 55222 W
        - infiltration                  - 14558 W
Specific heat load                      - 20.2 W/m3

This variant is in good agreement with the thermal requirements of the buildings, mentioned above.

Project - modeling

Comparison of the performance of the initial design and improved building construction can be prepared by modeling seasonally energy consumption of building. In table 5 is presented energy requirements for initial design, and in next tables 6 and 7 are presented energy needs for improved design.

Table 6 . Energy for heating - insulated building

Month	Monthly energy [kWh]	Heatng hours [h]
October	11583	679
November	21332	679
December	30996	744
January	34700	744
February	27960	678
March	24242	702
April	12300	492
Heating Season	163114	4520

Table 7 Energy for heating - insulated building and night window insulation

Month	Monthly energy [kWh]	Heatng hours [h]
October	11574	679
November	20720	679
December	28214	744
January	30749	744
February	25387	678
March	23051	702
April	12268	492
Heating Season	151764	4520

An important part of building energy simulation is a calculation of solar energy utilization by external elements. We have used program SOLAR to calculate solar gains through transparent elements of building. This program use data for solar radiation on a horizontal surface (or data for sunshine hours if solar radiation data is not available for a reference region), converts this data for vertical surface and reference orientation, calculates shading-transparency coefficient of windows and determines a solar energy penetrating the room. A calculation is made with hourly interval data (table.3) for representative day of all months in heating season.

The next table 8 shows daily energy summa of solar energy penetrating double glazed transparent element with east and west orientation:

Table 8. Daily solar energy gains trough double glazed window area [Wh/m2]

Total energy gains of solar radiation by windows for heating season are 39680 kWh

Thermal Comfort

The cooling load is composing on different gains that are acting in different periods in day. Their sum form a daily distribution on cooling load for the reference month. In Table 4 is presented the daily cooling load distribution for the building for the typical summer month - August.

The most effective way of protecting a building is to shade its windows and other apertures from unwanted direct sunlight. Careful organization of rooms according to their function can help the living space as cool as possible. When external air is cooler than the upper control unit, fresh air driven through the building by naturally occurring differences in air pressure can help to remedy this problem. In situation where the air inside a building is warmer than ambient air and cooling is required, the temperature difference or 'stack' effect can be used to expel the warm air from building.

The doors and windows, located on the eastern and western facade of the presented building, allow natural ventilation for cooling to be used. In hot dry summer climate for Sofia region, where night-time temperatures are low, cross ventilation at night is an appropriate method of removing heat accumulated in the building fabric during the day.

Indoor temperature variation in summer period (in winter also) is low, because of big thermal mass of building envelope (insolation of external walls is at outer side). Detailed calculation of indoor temperature variation is not made. This will be of interest in next investigations of thermal performance of the building.

Daylight

Introduction

While the light performance of residential buildings including the use of daylight, may not have as significant an effect on the amount of energy used as would be the case in office buildings, the quality of light is an important consideration for the comfort and well-being of occupants and the way in which the architecture of the spaces is modeled.

Description

Daylighting design involves the provision of natural daylight in the interiors of building to reduce or eliminate daytime use of electric lights, thereby offering sometimes substantial savings in energy use and consequent environmental damage ; and if skillfully executed, can provide heating and more pleasant living conditions.

Geometry

Good electrical lighting design and control can reduce energy use significantly, but a more fundamental and rewarding approach to the problem is to first design or modify the form of the building to allow it to admit and evenly distribute sufficient natural light to all of the occupied space. Often, in conventional buildings, there will already be more than adequate natural light close to the perimeter but this will fall off dramatically towards the core of the building, five or six metres back from the glazing, and occupants will try to correct any deficiency by switching on the lights.

The presented building has two external facades that provide natural daylight to the living space. Most of rooms have direct connection to the Daylighting sources - external windows.

Devices controlling daylight

Various devices are now available to capture daylight and direct it deep into buildings and to reduce excessive light levels near glazing, providing a more uniform spread of natural light. Some of these, such as atria, light shelves, roof monitors or clerestory lighting can have profound architectural design implications. Others such as prismatic glazing reflective blinds or shading systems can be more easily applied in the case of existing buildings. A wide range of specially treated glazing materials that can control the intensity and optical properties of natural light and heat flows is now available.

Results

The case for Daylighting in buildings has three strands: it can provide a healthier, more enjoyable indoor environment ; it can conserve the earth�s resources; and because it saves energy it saves money.

The following measures can help to avoid overheating:

- Solar control: to prevent the sun�s rays from reaching, and in particular entering the building.

- External gains control: to prevent increases in heat due to condition through the building skin or by the infiltration of external hot air.

- Internal gains control: to prevent unwanted heat from occupants and equipment raising internal temperatur�

- Natural cooling: to transfer excess heat from the building to ambient heat sinks, including: ventilation, where unwanted hot air is replaced by fresh external air at a lower temperature.

Conclusions

Heating load

Heat loss calculation for building (HC_LOAD program) gives us information about maximal requirements for heating installation. Results for different variants of walls' and windows' construction show that we can reduce installed capacity of heating plant with 35%, which is really economy of capital. Specific heat load is reduced from 31.7 W/m3 to 20.2 W/m3.

Energy requirements

Improving building design brings an important energy saves in building performance. About 75772 kWh is yearly economy of energy for heating by using insulated walls, improved windows' construction and night windows' insulation. This is 33% of total energy requirements for the building. If we add passive solar gains, energy savings become 115452 kWh or 50.7% of total energy requirements. This is considerable energy economy and it will influence financial decision for this building and all buildings in the dwelling complex.

Thermal comfort

Thermal comfort in building in winter season is supported by heating installation. Summer comfort in living space of building is maintained only with a passive means. This includes: movable shading devices (blind, curtains), control of internal gains, natural ventilation etc.

Daylight

The building has a big windows' area on the eastern and western facade. Those windows are sufficient to provide living space with natural daylight.

REFERENCES

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2. N.K. Bausal and M.S. Bhanderi, Comparison of the Periodic Solution Method with TRANSYS and suncode for Thermal Building Simulation, J. Solar Energy, 1, 9-18 (1996).
3. R.Festa and C.F.Ratto, Proposal of a numerical procedure to select Reference Year, J. Solar Energy, 1, 9-17 (1993).
4. T. Kosuda, NBSLD, the Computer Program for Heating and Cooling Loads in Buildings, Natural Bureu of Standards, Washington, 1976.
5. R.Lebens, Passive Solar Heating Design, APPLIED SCIENCE PUBLISHERS Ltd, London, 1980.
6. E.Jager, Solar Energy Applications in Houses, Pergamon Press, Oxford, 1981.
7. J.A.Duffie,W.A.Beckman, Solar Engineering of Thermal Processes, Jon Weley & Sons, New York (1980).