AleMac's profile - activity

Hi, I would have a further question about the FluidCooler model in EnergyPlus. I am trying to figure out the sizing appr

I am developing a simplified model of an air-to-water reversible heat pump in Modelica. The model is based on a curve-fitting approach for the calculation of the COP with data retrieved from the technical documentation of a heat pump unit. The COP is a function of evaporator and condenser temperatures. I am using a bi-quadratic function (as EnergyPlus does for example for chiller models)

My doubt refers to the cooling mode. In particular, I can't understand what exactly happens to the COP of the machine when the outside air temperature (temperature entering the condenser) goes below the water temperature (temperature leaving the evaporator). This situation can happen for example when a building needs cooling during a cold day.

Depending on the set of data that I use for the curve-fitting approach, I found two different behaviors: in graph1, the COP continuously increases

.

In graph2 the COP reaches a maximum approximately in correspondence with the value of the water temperature (it was 14 °C in this case)

Theoretically, whenever the outside air is lower than the water temperature, almost 100% free cooling could be achieved, however I am not really sure about the reality.

I hope you have some comments about this.

If the efficiency is 0.8, more energy will be consumed. But what if this boiler has an ideal efficiency of 1? This is basically what I modelled in my system. How would you call that energy? Ideal energy consumption?

My question refers only to the terminology, not to the modeling itself. What I would like to know is how you would call the energy input for this ideal source of energy. I will try to explain myself with an example. Let's assume I have a thermal zone served by a radiator that has to meet the set-point temperature in the room. The radiatior is connected to a boiler that heat up return water from the room. The energy consumed by the boiler depends by the efficiency of the boiler. If the boiler has efficiency 0.9, a certain amount of energy will be consumed.

Usually, in literature, the energy used by the boiler is called "energy consumption". But what if I replace the boiler with an ideal source of energy that just use the expression I showed in my first message (basically a machine with efficiency 1)? In this case it is not really "energy consumption", but I would say it is a kind of "ideal energy consumption". I hope I was more clear.

I will try to be more clear. First of all I am using Dymola (Modelica) as software. My question only refers to the terminology, not about the modeling itself. Imagine that in my model I have a thermal zone served by a radiator that has to meet the set-point temperature in the room. Let's assume that in this simple example there is no any ventilation system. Therefore we only have to meet the sensible heating loads. The radiatior is connected to a boiler. This bolier will use energy to heat up the water, and the energy consumed depends by an effciency curve.

However, since these three sources are not real machines (e.g. bolier, chiller etc), but are just ideal machines, I was wondering how I could call the required energy PS: at this stage I am not considering electricity for lighting and equipment.

Thanks for the answer. I should probably add that I have also implemented two other ideal sources for heating and cooling coil in the AHU. The AHU is connected to two secondary water loops, one for heating and one forr cooling. Therefore, in total, I have three ideal energy sources: 1) water circuit for both sensible heating and cooling 2) heating of air 3) cooling of air. In conclusion, the energy required by these three ideal energy sources to condition outlet water temperature and reach the inlet value can be considered as the total energy demand of the building.

Dear all, I have created a very simple ideal source of energy for building energy simulations, and I was now wondering: how would you define the energy required by a building when an ideal source of energy is responsible for heating/cooling of outlet water to reach the inlet water temperature? Basically, this ideal source of energy would just use the following expression: Q=m_dot x cp x (T_in - T_out) for heating Q=m_dot x cp x (T_out - T_in) for cooling

Could I define this as energy demand? Thank you in advance for your attention! Alessandro

Thanks for the exhaustive answer, Jason. Yes, parametric analysis would be an excellent method to find optimal shading controls in terms of energy vs. visual comfort. However, in my simulations, I am not considering daylighting and, at this stage of my work I am looking for very rough values. I think that the "balance point" you suggested for outdoor air temperature could be a good value. Regarding the insolation, maybe I could use a kind of average value of the warmest month (in this case is July).

location added to the question

@Jamie Bull -thanks, I have just added more details

I forgot to say that it is an office building.

In order to avoid overheating in the summer in office buildings, I would like to add a shading control that operates when solar radiation is above a defined set point (W/m2). However, this strategy would probably block welcome solar gains in the winter. Therefore I was thinking about a double shading control that takes into consideration also the outdoor air temperature. So, if outdoor air temperature > SetPoint1 & solar radiation > SetPoint2 then the shading device is lowered. Do you have an idea of typical values for SetPoint1 (°C) and SetPoint2 (W/m2)? I am working with EnergyPlus and Modelica, but I guess this is a general question. The building is located in Copenhagen (Denmark).

Dear all,

I have heard that it might be complex to model and simulate a large amount of thermal zones with Dymola. So far, I have been working with a two thermal zones model (using the LBNL Buildings library) and everything goes fine. I was wondering, which is the maximum number of thermal zones that you would suggest to use in a model? Is it only a matter of computational time or there are also modelling issues when having several thermal zones?

All the Best, Alessandro

@Amir Roth fixed

@Julien Marrec Those references seem very useful! Maybe I could try to use the set of equations described at page 630/1444 for the chiller and invert the procedure in order to get the condenser leaving temperature as set point (instead of original procedure where the set point is at the evaporator).

Julien, I am kind of familiar with EnergyPlus, but I could't find any explicit air-to-water heat pump model in the engineering reference document. I will have a look at the example file you mention. Thanks!

Indeed I am using the LBNL building library to develop my model, so I will definitely have a look at the package you suggest.

Thanks for your answer Julien, that is exactly why I couldn't understand, I thought it was an air-to-water heat pump! Do you know where I could find performance curves for a air-to-water heat pump?

I have just added some more info to my question

I am trying to model an air-to-water heat pump with Modelica. The heat pump should be able to heat water to a temperature between 20-23 °C during the whole year. A Boiler will assist the heat pump when outdoor temperature becomes too low in winter season.

Since I could not find any air-to-water heat pump model in the Modelica library that I am using, I decided to develop a simple model by myself and I am looking for performance curves. I have found this link that includes standard performance curves for heat pumps:

http://www.comnet.org/mgp/content/676...

If you look at the section "Electric Heat Pump Heating Efficiency Adjustment Curve" you will see that the parameter EIR_FT is function of two variables: Toadb and Tdb. Toadb is the outdoor air dr-bulb temperature, but unfortunately I cannot understand what Tdb actually refers to. In the description it says Tdb: The entering coil dry-bulb temperature.