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Predictive Pre-cooling Control For Low Lift Radiant cooling USING BUILDING THERMAL MASS. Motivation : energy and climate. Addressing energy, climate and development challenges Buildings use 40 % of energy and 70% of electricity 1

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motivation energy and climate
Motivation: energy and climate

Addressing energy, climate and development challenges

  • Buildings use 40% of energy and 70% of electricity1
  • Buildings represent low cost carbon emission reduction potential2
  • Most rapidly developing cities in cooling-dominated climates3
  • Increasing demand for A/C where grids are already stressed4

USDOE 2006. Building Energy Databook

IPCC 2007. Fourth Assessment Report

Sivak 2009. Energy Policy 37

McNeil and Letschert 2007. ECEEE 2007 Summer Study


Predictive pre-cooling control for low lift radiant cooling using building thermal mass can lead to significant sensible cooling energy savings.

  • What is a low-lift cooling system (LLCS)?
  • How is it implemented using building thermal mass?
  • How is predictive pre-cooling control achieved?
  • How significant are the energy savings?
low lift cooling systems llcs
Low lift cooling systems (LLCS)

Cooling strategy that lever-ages existing technologies:

  • Variable speed compressor
  • Variable flow hydronic distribution
  • Radiant cooling
  • Thermal energy storage (TES)
  • Pre-cooling control
  • Dedicated outdoor air system (DOAS)

…to save cooling energy:

  • Operate chillers at part load and low lift more of the time by predictive pre-cooling control
    • Night time operation
    • Load leveling
    • Radiant cooling
  • Reduce transport energy req’d to deliver cooling to a space
  • Efficient dehumidification
simulation of llcs shows significant cooling energy savings potential
Simulation of LLCS shows significant cooling energy savings potential

Simulated energy savings: 12 building types in 16 cities relative to a DOE benchmark HVAC system

Total annual cooling energy savings

  • 37 to 84% in standard buildings, average 60-70%
  • -9 to 70% in high performance buildings, average 40-60%

(Katipamula et al 2010, PNNL-19114)

llcs cooling energy savings in atlanta
LLCS cooling energy savings in Atlanta

Simulated total annual cooling energy savings:

  • in a medium size office building
  • in Atlanta
  • over a full year
  • with respect to a variable air volume (VAV) system served by a variable-speed chiller with an economizer and ideal storage
  • similar to a split-system air conditioner (SSAC) used as an experimental base line, with some differences

28 % annual cooling energy savings

(Katipamula et al 2010, PNNL-19114)


700 psia

Low lift vapor compression cycle requires less work





Vapor compression cycles shown under typical and low-lift conditions


Low-lift refers to a lower temperature difference between evaporation and condensation


Predictive pre-cooling of thermal storage and variable speed fans


T - Temperature (°C)

Variable-speed compressor adapts exactly and efficiently to conditions



Radiant cooling and variable speed pump







S - Entropy (kJ/kg-K)


LLCS operates achiller at low lift more of the time

where COP = f(Tx,Tz,Q) [kWth/kWe],

subject to just satisfying the daily load requirement:

are the simulation results real
Are the simulation results REAL?
  • Develop the pre-cooling control and experimentally test an LLCS
  • Optimize control of a chiller over a 24-hour look-ahead schedule to minimize daily chiller energy consumption by operating at low lift conditions while maintaining thermal comfort
  • Informed by data-driven zone temperature response models and forecasts of climate conditions and loads
  • Informed by a chiller performance model that predicts chiller power and cooling rate at future conditions for a chosen control
low lift chiller performance
Low lift chiller performance

Predictive pre-cooling control requires a chiller model to predict chiller power consumption, cooling capacity and COP at low-lift

To identify a chiller model under low lift conditions:

  • Build a heat pump test stand
  • Experimentally test the performance of a heat pump at low pressure ratios, which was later converted to a chiller for LLCS
  • Identify empirical model of chiller performance useful for predictive control
measured heat pump performance at many steady state conditions
Measured heat pump performance at many steady state conditions
  • Tested 131 combinations of the following conditions
  • To model chiller power, cooling rate, and COP as a function of all 4 variables
test results show expected higher cops at low lift conditions
Test results show expected higher COPs at low lift conditions

Low lift operation

COP ~ 5-10

Typical operation

COP ~ 3.5





empirical models accurately represent chiller cooling capacity power and cop
Empirical models accurately represent chiller cooling capacity, power and COP

4-variable cubic polynomial models

zone temperature model identification
Zone temperature model identification

LLCS control requires zone temperature response models to predict temperatures and chiller performance

  • Develop data-driven models from which to predict
    • Zone operative temperature (OPT)
    • The temperature underneath the concrete slab (UST)
    • Return water temperature (RWT) and ultimately chiller evaporating temperature (EVT) from which chiller power and cooling rate can be calculated
  • Assume ideal forecasts of outdoor climate and internal loads
  • Implement data-driven modeling on a real test chamber
existing transfer function modeling methods can be applied to predict zone temperature
Existing transfer function modeling methods can be applied to predict zone temperature

OPT = operative temperature

OAT = outdoor air temperature

AAT = adjacent zone air temperature

QI = heat rate from internal loads

QC = cooling rate from mechanical system

a,b,c,d,e = weights for time series of each variable

  • (Inverse) comprehensive room transfer function (CRTF) [Seem 1987]
  • Steady state heat transfer physics constrain CRTF coefficients
evaporating temperature is predicted from intermediate temperature response models
Evaporating temperature is predicted from intermediate temperature response models
  • Chiller power and cooling rate depend on
    • evaporating temperature, which is coupled to
    • return water temperature, and thus to the
    • state of thermal energy storage, in this case a radiant concrete floor
  • Predict concrete floor under-slab temperature (UST) using a transfer function model
  • Predict return water temperature (RWT) using a low-order transfer function model in UST and cooling rate QC
  • Superheat relates RWT to evaporating temperature (EVT)
data driven models identified for a test chamber with a radiant concrete floor
Data-driven models identified for a test chamber with a radiant concrete floor

Temperature sensors: OPT, OAT, AAT, UST, RWT

Power to internal loads: QI

Radiant concrete floor cooling rate: QC


Models validated based on accuracy of predicting different data 24-hours-ahead

Sample validation temperature data

Sample validation

thermal load data


Transfer function models accurately predict zone temperatures 24-hours-ahead

Operative temperature (OPT)

Under-slab temperature (UST)

Return water temperature (RWT)

Root mean square error (RMSE)

for 24 hour ahead prediction of validation data

OPT RMSE = 0.08 K

UST RMSE = 0.15 K

RWT RMSE = 0.84 K

optimization minimizes energy maintains comfort and avoids freezing the chiller
Optimization minimizes energy, maintains comfort, and avoids freezing the chiller
  • rt = electric rate at time t, or one for energy optimization
  • Pt = system power consumption as a function of past compressor speeds and exogenous variables
  • = weight for operative temperature penalty
  • PENOPTt = operative temperature penalty when OPT exceeds ASHRAE 55 comfort conditions
  • PENEVTt = evaporative temperature penalty for temperatures below freezing
  • = Vector of 24 compressor speeds, one for each hour of the 24 hours ahead

Perform optimization at every hour with current building data and new forecasts

Pattern search initial guess at current hour

24-hour-ahead forecasts of outdoor air temperature, adjacent zone temperature, and internal loads (OAT, AAT, QI)

Pattern search algorithm determines optimal compressor speed schedule for the next 24 hours

Operate chiller for one hour at optimal state

experimental assessment of llcs
Experimental assessment of LLCS

Prior research shows dramatic savings from LLCS, but

  • Based entirely on simulation
  • Assumes idealized thermal storage, not a real concrete floor
  • Chiller power and cooling rate are not coupled to thermal storage, as it is for a concrete radiant floor

How real are these savings?

What practical technical obstacles exist?

building and testing a low lift system
Building and testing a Low-Lift System
  • Build chiller by modifying an inverterheat pump outdoor unit
  • Install a radiant concrete floor in MIT and Masdar test rooms
  • Implement the pre-cooling optimization control
  • Test LLCS under a typical summer week in Atlanta (and next Phoenix) subject to internal loads—then in real AD weather
  • Comparthe LLCS performance to a baseline system - a high efficiency (SEER~16, SCOP~4.7) variable capacity split-system air conditioner (SSAC)











LLCS and SSAC use the same outdoor unit



LLCS provides chilled water to a radiant concrete floor (thermal energy storage)














Chiller/heat pump

Radiant concrete floor


Tested LLCS for a typical summer week in Atlanta subject to standard internal loads

Atlanta typical summer week and standard efficiency loads

Based on typical meteorological year weather data

Assuming two occupants and ASHRAE 90.1 2004 loads

Run LLCS for one week *(after a stabilization period)

Run split-system air conditioner (SSAC) for one week*

Compare sensible cooling only

Mixing fan treated as an internal load

Repeat for Phoenix typical summer week, high efficiency loads – to be completed after climate chamber HVAC repairs


Outdoor climate conditions

Internal loads

Atlanta test

Phoenix test


LLCS ENERGY SAVINGS relative to SSAC in Atlanta subject to standard loads

Similar to simulated total annual cooling energy savings, 28 percent, by (Katipamula et al 2010)

  • Latent cooling is deducted by measuring condensate water from the SSAC, calculating the total enthalpy associated with its condensation, and dividing it by the average SSAC COP over the week.
  • Assuming no latent cooling by the LLCS

Predictive pre-cooling control can be applied to other systems to achieve low lift

  • Simulated the performance of predictive pre-cooling control on the SSAC and with radiant ceiling panels
  • Significant savings potential for predictive control on other systems
future llcs research
Future LLCS research

Refine LLCS methods

    • Determine evaporating temperature without measuring under-slab concrete temperature
    • Refine temperature response model identification methods, e.g. real-time model identification with updated training data
    • Simplify and improve the pre-cooling optimization and control
  • Combine concrete-core with direct cooling (e.g. chilled beams) and adapt the predictive control algorithm
  • Perform testing subject to actual outdoor conditions at MASDAR
  • Install and test LLCS in a real building (medium size office)
  • Pre-cooling control for other LLCS configurations
future of llcs in real buildings
Future of LLCS in real buildings

Concrete-core and radiant systems gaining market share, and familiarity among architects and engineers (primarily in Europe)

Automation systems are becoming more prevalent/sophisticated

Capital cost savings for LLCS in medium office buildings,

-0.58 $/sqft incremental cost relative to $7.91/sqft base cost1

Adapt components of LLCS to existing buildings and different new and existing building types, e.g.

  • Direct cooling combined with active or passive thermal storage
  • Radiant concrete-core using a “topping slab” for existing buildings
  • Adapt low-lift predictive control to existing concrete-core buildings

1. Katipamula et al 2010, PNNL-19114

  • Detailed data on the performance of an inverter-driven rolling-piston compressor heat pump over a wide range of conditions including low lift, over a capacity range of 5:1
  • Methodology for integrating chiller models and zone temperature response models into a pre-cooling optimization algorithm for controlling LLCS with real building thermal mass
  • Experimental validation of significant LLCS sensible cooling energy savings relative to a state-of-the-art split system air conditioner (SEER 16), 25 percent in Atlanta with standard efficiency internal loads. >50% clearly achievable.