Pergamon Renewable Energy, Vol.5, Part IL pp. 1193-1201, 1994

Elsevier Seimge Lal Printed in Gt~t Britain

0960--1481/94 $7.004.0.00

THE UTIIIqATION OF OPTIMAL DESIGN AND OPERATION STRATEGIES IN LOWERING THE ENERGY CONSUMIrI'ION IN OFFICE BUILDINGS

Peter Simmonds I.Eng.,ACIBSE, Member ASHRAE t

An optimal Heating, Ventilating and Air-Conditioning (HVAC) system operation strategy requires the best combination of temperatures and flow quantities, in order to minimise the overall cost .The efficiency of the interaction between the building and its HVAC system while maintaining comfort conditions are improved to enhance the dynamic per- formance and reduce peak loads.Using dynamic simulation programs configurations were simulated that resulted in an optimum design being achieved based on comfort conditions not temperatures.This paper shows how different constructions ,glass types and solar shading were simulated, resulting in an optimal system design and operating strategy for a comfortable indoor climate throughout the year.

INTRODUCTION Air-conditioning systems are the fastest growing of all building costs. Mainly due to the decentralisation of computer centres,(whieh were heavily air-conditioned). The necessity of supplying ventilation air into the office space has been extortionated in order to remove heat produced by modern day information technology equipment.Heating, cooling, lighting fans and pumps make up a major portion of the total energy consumption for office buildings.A buildings internal energy, produced by lighting ,equipment and occupants may be used to beneficially reduce energy costs (Simmonds 1993). The energy needed to provide comfortable working conditions can be significantly reduced by careful design. Buildings should interact dynamically with the environment to take advantage of free energy and provide comfortable living and working spaces. Designs show that the environ- ment can be used as an architectural generator and how the building form can be moulded in response to environmental pressures. Building operation strategies can greatly affect occupant comfort and energy use, therefore building simulation provides the system designer (and Architect) with detailed information about the dynamic behaviour of the building and the interaction between the building and its HVAC system. With this infer- marion the Building services designer can then optimize the capacity and operating sched- ules of the HVAC system to take advantage of the thermal interaction of the building and utility rates and tariffs.Control strategies are optimized to minimize the operating cost (i.e. energy and demand charges) while controlling the HVAC system to meet comfort criteria in the occupied zone. In the optimal strategy ,the building space is controlled within comfort limits PMV+/- 0.5 (Fanger 1971,ISO 7730) throughout the occupation period.Recent studies by Simmonds (1992,1993) demonstrated that significant energy and demand savings were possible using an optimal strategy to minimise the total cost of HVAC operation.R is possible to improve comfort conditions and reduce energy consumption (Simmonds 1993). This work shows effective control methods for utilising building energy consumption and the extent to which these methods can reduce operating costs in commercial buildings.

~Peter Simmonds is a Research and Development Engineer with RTB Van Heugten, Groningen .The Netherlands.

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Dynamic ontimization The optimal control of a system takes advantage of the harmony of the building, its architecture, its materials and its systems, and involves minimizing the integral of operat- ing costs over a specified period of day(e.g, a day) whilst satisfying the required comfort conditions (PMV +/- 0,5) The operating costs at any given point in time is equal to the product of the power consumption of its systems (heating and cooling) and the costs of fuels. (gas or electrie).The constraints include both required comfort conditions for the building (PMV +/- 0,5) and limits on the operation of equipment (e.g. capacity). The optimal solution is a trajectory of control throughout the specified optimization period. The control variables include setpoints derived from PMV conditions,the approach used in this study is practically identical to that originated by Braun (1990) involves discretizing the cost function in time and applying a non smooth optimization minimize the sum of costs over the specified time.The optimal control problem starts with a statement of the cost function.The cost function includes costs of energy used by a HVAC system for a space and the comfort cost of the space occupants. The ability of a control strategy to improve comfort conditions, reduce energy costs and usage is measured by defining a cost func- tion for the HVAC and building systems. Because these systems are inherently transient, an integral cost function is used to account for costs over a period of time. For the HVAC and building systems the cost function J is represented by

K

J = ~ (R (IO + P'(Pmei~ , f(k)))

for all (K) with respect to PMV, (k) (k= 1 ,K) subject to

PMV6 ~tn (X3 < VtCCs (I9 ~ PMVs max (K)

The form of the cost function may differ depending upon the type of building under construction.The operating costs in eq. 1 and the comfort of the occupants in the building are evaluated by determining the energy usage by the HVAC systems and the environ- mental conditions within the space. Heat is transferred by conduction through the building envelope and partition walls;by convection,causing heat exchange between surfaces, by radiation in the form of solar transmission through transparent parts of the building envel- ope and in the form of long wave radiation exchange between surfaces,each of which must be treated separately.The configuration of perimeter walls and the percentage of glazing in the external and internal walls are usually a weak point for the thermal environment.The heat losses or gains of a building generally take place at the borders between the indoor and outdoor climates, that is to say, the building shell, which consists of the outside walls and glass areas, the roof, and the floor. To keep these resulting heat losses at a minimum, it is necessary to keep the borders to a minimum, which requires a unique building, for this reason, the Aolv relationship is important, that is, the relationship between the heat-exchanging encasing area Ao (the shell) and the enclosed volume v (the gross vol- ume). The U-value, the transparency and the reflectance are features that have consider- able effects on the thermal environment.The path of sunlight must be tracked in the space which modifies the MRT (at each hour) through direct solar radiation( if the analysis point is not shaded).It must not be forgotten that the qualities that are desirable for summer conditions (low transparency and low reflectance)lessen the heating load during the win-

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ter.Climate windows are double glass windows complemented by a third layer on the inside.Air is extracted from the room via the opening between the second and third layer of glass. Solar heat which then enters the window is directly absorbed by the extract air and does not enter the room. The advantage of this system is that the temperature of the inside glass has nearly the same temperature as the room summer and winter, this greatly improves the comfort levels in the room. As the solar heat is absorbed by the extract air it provides energy for heat exchange equipment in the air handling unit. This is accom- plished by specifying the performance characteristics of the HVAC system, the weather conditions, the occupancy schedule and the buildings architectural characteristics.

The optimal control of a system will take full advantage of acceptable comfort condi- tions(PMV +/-0.5).Figure 1 shows the optimisadon of the PMV calculation. During the winter period the lowest acceptable PMV value is -0.5,the corresponding room dry bulb temperature could be lower(18-19°C) than recommended room dry bulb temperatures for office buildings ( 20-21°C).

BUILDING LOADS Internal loads can be in the order of 30 to 45 W/m s ( floor area), glazing is usually about 30% of the facade, some 3m z, for a 20m z floor space. A large percentage of these internal gains (between 60 and 70%) is in the form of electromagnetic radiation and is absorbed directly by internal surfaces prior to being convected to air space.Typical modern day glazing can filter out some 70% of the solar heat gain. Assuming a solar load of 650 W/m 2 , about 220 w enters the space. This compared to the internal load of between 600 to 900 w, is only 20% of the total cooling load. At this point the influence that the glazing surface has on the MRT must be considered, especially when using Low -E glazing for example, the thermal properties of the glazing reduce the thermal load, but the increased temperature of the inside pane ,increases the MRT. During the winter period heat losses from the space are about 200 W by an outside air temperature from -5°C.Therefore heating is not important, cooling has priority.Using a carefully designed system the internal energy can be utilised to reduce HVAC energy costs.

Substituting the dry bulb temperature for dry resultant ,environmental or operative tem- peratures involves radiant temperatures into the criteria, but does not offer the optimum solution.Optimal control techniques will optimise energy consumption when bringing a room temperature to the required temperature after night set beck.However ,radiation heat exchange of the surroundings could produce a cold sensation producing an uncomfortable climate.When targeting temperatures for the beginning of the occupation period,impeding room loads(lighting,occupants and equipment) are not considered.These internal heat loads will increase indoor temperatures during the first hours of occupation,this is a non optimal energy usage.The solution to this problem creates a great deal of discussion ,namely, keeping the indoor temperature above 18°C during non occupied periods ,using PMV to determine room conditions at the beginning of occupancy and the use of predic- tive controls or simulation to indicate conditions during the occupancy period.During summer operation ,the PMV is allowed to border the maximum constraint of +0.5.The corresponding dry bulb temperatures could be higher than recommended (24-25°C),this strategy would ensure that the minimum of energy be consumed in controlling the space environment.If some form of variable plant is used to condition the space then the energy requited to condition the space during part load would be lowered,because the PMV is

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being controlled to border maximum limits Using PMV optimisation techniques the dynamic interactions are constantly moni-

tored.Alternative options regarding building construction and configuration details can be presented and analyzed. This enables designers to quickly re-assess Architectural design changes. If for example ,an alternative glazing is suggested.Usually only the U-values are compared ,but reflectance and light transmission factors are often overlooked.This could result in a non optimal or bad design.lmagine what the consequences would be if the solar transmittance is higher ,then the temperature of this glazed surface will increase which in turn will effect the radiation heat exchange and disrupt the thermal environmental bal- ance.The same is applicable to changes in the construction of perimeter or internal walls,roofs and floors, a constant PMV assessment throughout the design process ensures a harmonious integrated design. Plant characteristics

Previous studies by Simmonds(1992,1993) showed that primary low rate ventilation air (2 air changes per hour(ACH)) together with VHV units (figure 2)provides an energy effi- cient HVAC system. Supplying air at low velocity in the space will create a pool of fresh air that will pour evenly through the space. When air comes in contact with heat sources, such as equipment and people, it will rise up, ventilating and removing heat. This air is collected at high level and cooled by the VHV unit and re-enters the space where its physical properties circulate the a through the room. The amount of secondary circulation is dependant upon the temperature differential between the air in the lowered ceiling and the surface temperature the VHV unit. With this system the circulation rate increases as the heat load in the space increases. Free cooling mode Figure ~ shows how the VHV is connected to the mixing pipe of the combined heat-

ing/cooling coil. Convective heat is extracted from the room by the VHV units, water running through the VHV units is heated from 13°C to 17°C.This water at 17°C is then mixed with 13°C water returning from the combined coil. The VHV water is cooled to 13°C, the return water from the combined coil is a constant 13°C.The exiting air tem- perature from the combined coil is controlled at a constant 13°C.This primary ventilation air can then be heated to 17°C ,by means of a reheat coil, from recovered sources.A three-way control valve is used to control the 13°C supply water temperature to the VHV units. Results Figure ] shows the difference in room loads through 1 m 2 of different glazing configur- ations.The best results are achieved with climate windows. During winter periods solar energy can be transferred back to air handling unit by the extract air flowing through the climate window. Figure ~. shows the dry-bulb, dry resultant (operative) and mean radiant (MRT) tempera- tures for one of the glazing configurations. It can be seen that the dry-bulb temperature is between 24-26"C throughout the occupation period. The MRT increases to about 28°C. For glazing with inside solar shading or no solar shading then the temperature of the room side material will rise to a temperature of about 35-40"C, which increases the MRT and decreases the comfort conditions. Figure ~" shows the PMV results for 6 configurations after optimisation for a south orien- tated office during a july day.The different configurations are maintained within the maximum permissable limit of +0.5 PMV.At the beginning of the occupation period the

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PMV values are low, between -0.6 and -0.3. During the first hour of occupation the PMV values rise rapidly, this increase is produced by the loads (mostly internal). figure 6 shows the corresponding room temperatures (dry-bulb) for the PMV values shown in figure ~. Only one of the configurations has temperatures around 25*(2, the other configurations have temperatures between 26 and 27°(2. Clearly the cooling loads necess- ary in producing these effective PMV values are lower than conventional cooling necess- ary in producing room temperatures of 25*(2. Conclusions

The object of this research was to investigate the design simulation and operation of a comfortable indoor climate for a standard office. The results clearly show that this has be achieved. The impact of dynamic simulation has been shown to be very useful. A tech- nique has been utilized for the design and operation of a building and its installations which is easily implemented on a computer enabling an optimal strategy to be determined in advance from a knowledge of the various building interactions. This method affords a high degree of flexibility in the choice of constraints. It provides useful means of improv- ing present design aids for the sizing and operation of a building and its plant under transient conditions. Several different facade constructions were simulated but the differ- ence between them was minimal. The test results did show that significant results could be obtained with different glazing configurations.When comparing various glazing alternatives the emphasis not lie on solar heat gains alone, but on the complete thermal perform- ance.Reducing solar loads can be accomplished by special types of glazing, but due to their specific characteristics the MRT in the space is increased, which decreases environ- mental comfort. Internal solar shading can also be used as method of reducing solar heat gain, but will produce an increase in convection heat energy in the space. However, this convection heat will rise through the space where it is cooled by the VHV unit. Climate windows produce the best results, summer and winter, making it possible to produce high IT values, decreasing plant size and increasing the performance of the HVAC plant. The VHV units provide a simple and effective means of removing unwanted heat from a space. As there are no motofisod parts in the unit, sound levels are not affected also there are no penalties due to high pressure losses( as with VAV terminal devices).The VHV units provide an excellent means of free cooling, this point must not be overlooked, considering that internal loads of approximately 35W/m 2 are constantly being released into the space, during occupied hours. The heat energy is recovered and transported to the AHU where it heats the primary ventilation air, up to outside temperatures of 12°C. This equates to some . ~ . . . h o u r s of reduced plant energy consumption. The PMV optimisation for controlling space conditions is an excellent means of increasing comfort levels ,as the space is continually controlled between PMV+/- 0.5, also the energy consumption is decreased.Another advantage of the VHV unit with low ACH primary ventilation was the individual zone conditions it created. If a particular office or part of an office had a lower internal load than simulated, then the room temperature would remain within the comfort conditions, the convective heat gain in the lowered ceiling would be minimal,and therefore the VHV unit would not be ufilised and consequently, no ancillary cooling energy would be required.The system is a self regulating system without using a complicated control strategy to carry out the simple tasks required. Operating an office under comfort conditions and not temperatures clearly shows that the energy necessary to maintain the required conditions is less than by conventional systems.

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References 1. ROOM - "A method to predict thermal comfort at any point in a space" ,Copyright OASYS Ltd. developed by ARUP Research and Development,London, England. 2. Simmonds P., 1992, "The design and simulation of a comfortable office space", Building Envelope Design.ASHRAE/DOE/BTEC Conference.Clearwater Beach ,Florida. 3. Simmonds P., ~A Building's Thermal Inertia', CIBSE National Conference 1991. 4. Simmonds P., "The Utilization and Optimization of A Building's Thermal Inertia in Minimizing the Overall Energy Use', ASHRAE Transactions 1991 V97 Pt2. 5. Simmonds P.,"Thermal comfort and optimal energy use", 1993,ASHRAE Transactions 1993 V99 Ptl. 6. Fanger P.O., 1972, "Thermal comfort analysis and applications in environmental engineering", Mc Graw-Hill, New York. 7. ISO 7730 1984, "Moderate thermal environments - determination of the PMV and PPD indices and specification of the condition for thermal comfort. ~, International stan- dard ISO 7730, International Organisation for Standardization. 8.ASHRAE 1989, ASHRAE handbook-1989 fundamentals, Atlanta: American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. 9.Braun, J.E., 1990, "Reducing energy costs and peak electrical demand through optimal control of building thermal storage.", ASHRAE Transactions 96(2).

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consumption i t stage k (eq, weather and internal gains)

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Fig. 2 SCHEMATIC FREE COOLING

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Fig. 3 COMPARISON OF DIFFERENT GLAZING. SOUTH ORIENTATION, JULY.

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Fig. 4 Dry bulb, Dry Resultant, and Mean Radiant(MRT) for a Typical Office Configurations

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FIG.~'SHOWS THE PMV RESULTS AFTER OPTIMISATION FOR 6 DIFFERENT BUIDING CONFIGURATIONS FOR A JULY PERIOD

0,6 ...............................................................................................................................................................................................

0,4 .................................

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8 9 10 11 12 13 14 15 16 17 18

TIME

I • CBM ~ CHIP * NCR ~ FGDP A KLM - . - -o - - - AZG ] J

FIG..~..SHOWS THE TEMPERATURE RESULTS FOR THE DIFFERENT BUILDING CONFIGURATIONS AFTER PMV OPTIMISATION

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