Stig-Inge Gustafsson

Optimisation of insulation measures on existing buildings

In Sweden, the activity on building new residences has been decreased for a number of years. The building stock as an average has therefore become older and in the future it will be subject for refurbishment. This paper deals with how to optimise retrofit measures, i.e. how to act in order to minimise the Life-Cycle Cost (LCC) of a building. Insulation measures are emphasised but also other retrofits are dealt with such as changing the heating system. It is shown that the heating system has a vital influence on the optimal amount of extra insulation which is to be applied. District heating is common in Sweden at least for larger buildings such as multi-family block of flats sited in urban areas. The tariffs for district heat must therefore be properly addressed in order to find out if extra insulation is profitable or not. As an example the Navestad area in Norrkoping is used. This residential area is now the subject for extensive retrofitting.

Life-cycle cost minimization considering retrofits in multi-family residences

Abstract When a building is to be renovated it is important to implement the optimal retrofit combination. If this strategy is neglected it might not be profitable to change the building in order to improve it as an energy system. This paper deals with energy retrofits and how the strategy can be optimized considering one specific building. The best solution is found when the life-cycle cost for the building is minimal, and building envelope, ventilation and heating system retrofits are combined. In order to solve the problem, a mathematical model, OPtimal Energy Retrofit Advisory (OPERA), has been developed. Energy balance calculations are used in which the free energy from solar radiation and from appliances is taken into proper account. The interaction between different retrofits is emphasized. Provided that the optimal solution is implemented, the retrofits in the combination will have a minor interaction which, for most cases, could be neglected. This will also imply that the order of implementation is of no, or minor, importance. A case study for a real building sited in Malmo, Sweden, and a sensitivity analysis for some critical input parameters are discussed.

Mixed integer linear programming and building retrofits

When a building is subject for refurbishment it is important to add only such measures that will reduce the Life Cycle Cost (LCC), for the building. Even better is to add measures that will, not only reduce the cost, but minimise the LCC. One means for such an optimisation is to use the so called Linear Programming (LP), technique. One drawback with LP models is that they must be entirely linear and therefore two variables cannot be, for example, multiplied with each other. The costs for building retrofits are, however, not very often linear but instead ‘steps’ are present in their cost functions. This calamity can, at least to a part, be solved by introducing binary integers, i.e., variables that only can assume 2 values, 0 or 1. In this paper it is described how to design such a Mixed Integer Linear Programming (MILP), model of a building and how different cost elements of the climate shield influence the optimal solution.

Optimisation and simulation of building energy systems

The Mixed Integer Linear Programming (MILP) technique is a useful tool for the optimisation of energy systems. However, the introduction of integers in linear models results in a severe drawback because the ranging process is no longer available. Therefore, it is not possible to study what happens to the solution if input data are changed. In this paper, we compare a MILP model of a building with a simulation model of an identical case. Both models describe a building with a number of possible retrofits. Using the MILP technique, the optimal retrofit strategy is calculated, after which certain input data are changed. The optimisation results in the lowest possible Life-Cycle Cost (LCC) of the building, and the paper describes how much the LCC will change if the property owner chooses other solutions. An increase in a particular data value may cause the LCC to increase or decrease. It may also be unchanged. Only a few data reduce the LCC when their values are increased.

Sensitivity analysis of building energy retrofits

When a building is refurbished, energy conservation measures might be profitable to implement. The profitability depends, among other things, on the electricity and district-heating tariffs, the unit price for oil, etc. The cost for the retrofit is of course also important as well as the influence of the retrofit on the demand for heat in the building. By the use of a Mixed Integer Linear Programming model of a building, a number of different optimal retrofit strategies are found depending on the energy cost. The result shows that the Life-Cycle Cost for the building is subject only to small changes as long as the optimal strategies are chosen. Most important is the heating system, while building retrofits such as added insulation, are too expensive to take part in the optimal solution.

Optimal heating-system retrofits in residential buildings

The optimal heating-system-retrofit strategy for existing buildings differs due to varying prices of energy, building and installation features, climate conditions, etc. We have examined a test building situated in Linkoping, Sweden. By using the OPERA model, we were able to arrive at the optimal retrofit strategy, which includes a ground-coupled heat pump using electricity to run the compressor. Unfortunately, the price of electricity differs according to the time of day, month, etc. These variations are not included in the OPERA model. In OPERA, the price should be divided into 12 segments, one for each month of the year since climate data are divided in this manner. Fine tuning of a dual-fuel system (an oil-fired boiler handles the peak load and a heat pump the base thermal load) is optimized using the Mixed Integer Linear Programming (MILP) method. Adding a hot-water accumulator also makes it possible to use low electricity prices for space and domestic hot-water heating. This system competes in the model with traditional heating devices such as district heating. The optimal method of heating the building was found for using the heat pump alone.

Heat accumulators in CHP networks

In a Combined Heat and Power (CHP) network, it is sometimes optimal to install a device for storing heat from one period of time to another. Several possibilities exist. If the electricity demand is high, while at the same time the district heating load is too small to take care of the heat from the CHP plant, it could be optimal to store heat from peak periods and discharge the storage under off-peak. It might also be optimal to store heat during off-peak and use it under the district heating peak load. The storage is then used for decreasing either the district heating demand or for decreasing the electricity load used for space heating. The paper shows how a mixed integer program is developed for use in the optimization process. As a case study, the CHP system of Malmo, Sweden, is used. Further, a sensitivity analysis is elaborated in order to show how the optimal solution will vary due to changes in certain input data.

Optimization of building retrofits in a combined heat and power network

We describe a mathematical model of a linear program for optimization of the following: the use of purchased and produced electricity (for power and for heat), the fuel mix in a district-heating plant, and implementation of energy-conservation measures in the building stock of the Malmo municipality. We find that energy retrofits are not profitable when compared to the cost for producing or purchasing more electric power and heat. The reasons for this result are, the low cost of electricity purchased from the national grid even under peak conditions and the exploitation of waste heat generated at the district-heating plant.

Furniture design by use of the finite element method

The design of structural members of furniture is almost never the subject for mathematical considerations. Instead, the designer rests on empirical experience and constructs for example chairs with dimensions of structural members based on tradition and aesthetic reasons. By the much more common use of computers it is nowadays possible to use modern finite element programs in various stages of the design process. In this paper we show how already simple calculations lead to a totally different design of a chair. We also emphasise the need for more research on wood in “furniture size” and not only as part of building structures. There is no need for “triple-security” values when a chair is to be designed but instead it is possible to balance on the edge of the mechanical strength in the wooden members. Further, it is possible to use only wood details where no knots, or other errors are present. This will lead to substantially thinner members in the wooden chair.ZusammenfassungDas Design von Möbelelementen ist nur selten Gegenstand mathematischer Überlegungen. Stattdessen stützen sich die Designer auf Ehrfahrungswerte und die Konstruktion von Einzelstücken aufgrund traditioneller oder ästhetischer Betrachtungen. Mit Hilfe von Computern ist es jedoch leicht möglich moderne FEM-Programme für die verschiedenen Entwicklungsschritte eines Designs zu nutzen. Wird betonen hier auch die Notwendigkeit weiterer Grundlagenforschung für Abmessungen im Möbelbereich nicht nur bei Baukonstruktionen. Für die Stuhlkonstruktion ist auch keine Dreifach-Sicherheit nötig, sondern man kann die mechanische Festigkeit der Konstruktionselemente voll ausnutzen. Zudem ist es möglich, durch Vorsortieren typische Holzfehler ganz unberücksicht zu lassen. Das wiederum ermöglicht schlankere Einzelteile für das Stuhldesign.