Radiant cooling and thermal comfort

Dr Simos Oxizidis simeon.oxizidis@ucd.ie

University College Dublin Meeting01 / 12 /2010

Heating and Cooling Terminal Units

High temperature cooling hydronicradiant systems are mainly building integrated (embedded) systems, in the form of radiant building surfaces like walls, floors or ceilings.

Heating & Cooling Terminal Units

Heating Mode TemperatureThermal Energy

Carrier

Radiant systems

Convective systems

Hybrid systems

Low Temperature

systems

Medium Temperature

systems

High Temperature

systems

Air systems

Steam systems

Water systems

(hydronic)

Refrigerant systems

Electric systems

Direct fired systems

High Temperature Cooling Hydronic Radiant Systems

Provision of better thermal comfort conditions by removing heat mainly by means of radiation.

Elimination of the local thermal discomfort causes which are usually related to the operation of convection cooling systems.

Absence of noise and draughts due to fan operation, resulting in discrete conditioning of spaces.

Comfort can be achieved with higher air temperatures (by decreasing the mean radiant temperature) thus, limiting air exchange heat gains.

Use of high temperature cooling enables the efficient use of plant equipment (air to water heat pumps) with increased overall energy efficiency.

By thermally activating the building elements, the thermal mass of the building’s envelope can be utilised for cooling, resulting in lower peak loads and lower equipment capacity. In addition, the building’s thermal mass can be used as thermal energy storage facility.

Thermal Comfort

There are two distinct ways of estimating comfort in a space:

(a) The deterministic route of Fanger’s PMV-PPD model and all its successors, derived from laboratory based research and

(b) the stochastic approach described by the adaptive comfort models that arose from field based research. The latter is more suited for evaluating thermal comfort conditions in free running, naturally ventilated buildings, where no mechanical cooling is used.

Thermal Comfort – The Deterministic Route

0 Neutral (Comfort)

+1 Slightly Warm

+3 Hot

+2 Warm

-1 Slightly Cool

-2 Cool

-3 Cold

1

10

100

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

PP

D (

%)

PMV

PMV (Predicted Mean Vote) is a function of

Air temperature

Human activity

Clothing type

Radiant temperature

Relative humidity

Air speed

Related with PMV is the PPD index (PPD –

Predicted Percent of Dissatisfied people)

Thermal Comfort – The Deterministic Route

PMV index in a room (limits for 10% of dissatisfied people – -0.5≤PMV≤

+0.5)

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

10 13 16 19 22 25 28 31 34 37 40

PM

V

Ambient Air Temperature (oC]

.

ASHRAE Standard 55 comfort area (limits for 10% of

dissatisfied people)

0

0.004

0.008

0.012

0.016

0.02

22 23 24 25 26 27 28 29 30

Hu

mid

ity

Rat

io

Operative temperature (oC)

0.5 clo (clothinginsulation

of occupants)

Thermal Comfort – The Stochastic Approach

The fundamental assumption of the adaptive approach is expressed by the adaptive principle: If a change occurs such as to produce discomfort, people react in ways which tend to restore their comfort.

There is a comfort indoor temperature that is linearly correlated with the outdoor temperature

16

18

20

22

24

26

28

30

32

5 10 15 20 25 30 35

Mean Monthly Outdoor Temperature [oC]

Ind

oo

r O

pe

rative

Te

mp

era

ture

[oC

]

90% acceptability limits

80% acceptability limits

ASHRAE Standard 55 acceptable operative temperature ranges for naturally conditioned

spaces

Test Cell Simulations

A model of an test cell was simulated for a four month cooling period (June to September) under the climatic data of Thessaloniki, Greece.

Simulation Conditions – Climatic Data

Daily Average Global Horizontal Radiation [Wh/m2]

Monthly Average Global Horizontal Radiation [Wh/m2]

Simulation Conditions – Climatic Data

Hourly Dry Bulb Temperature for the cooling period [oC]

Simulation Conditions

The U-values of walls, floor, roof and window are 0.33, 0.3, 0.26 and 2.75 W/m2K respectively

For the cell was assumed a constant ventilation of 0.9 ACH. It is occupied by two men who emit 126 W (1.2 met) each, 60% by radiation. The insulation of their clothing is 0.5 clo.

The internal heat gains by electric equipment are 5 W/m2.

For the thermal comfort (PMV index) calculations the air velocity in the cell was assumed 0.15 m/s during the heating season and 0.25 during the cooling season.

The temperature of the thermostatic control was set to 25oC.

Systems Compared

The following terminal units were compared:

Ideal conditioned air (PA)

Fan Coil (FC)

Radiant floor

Heavyweight construction (RF)

Lightweight construction (RFG)

Radiant ceiling (RC)

Radiant wall (RW) – south and north wall only

Heavyweight radiant floor and fan coil (RF FC)

Systems Compared – Radiant Units

a) Radiant Floor Heavyweight construction (RF)

b) Radiant Floor Lightweight construction (RFG)

c) Radiant ceiling (RC)

d) Radiant wall (RW) –south and north wall only

a) b)

c)

d)

Control Strategy for the radiant systems

Chiller

Pump

Supply

Mixing valve

Shut off valves

Return

Limiter

Central

control

unitOutside

temperature

Temperature -

Humidity

Room

sensor

Room

sensor

Local control

unit

Valve

ManifoldFloor

temperature

Control of the Systems

Control of the Systems

Temperature control of the supply water for the fan coil

6

7

8

9

10

11

12

13

15 20 25 30 35 40

Sup

ply

co

ld w

ater

te

mp

erat

ure

[οC

]

Outdoor Temperature [οC]

10

12

14

16

18

20

22

15 20 25 30 35 40

Sup

ply

co

ld w

ater

te

mp

erat

ure

[οC

]

Outdoor Temperature [οC]

0

20

40

60

80

100

24.5 25 25.5 26 26.5

Sup

ly c

old

wat

er f

low

[%

]

Zone Temperature [οC]

Temperature control of the supply water for the radiant surfaces

Flow control of the supply water for the radiant surfaces

Scenarios

Beyond the comparison determined by the above simulation conditions (reference scenario DBT) four other scenarios were investigated:

Controlling the indoor operative temperature (scenario OPT)

Controlling the internal thermal conditions by using the PMV (according to Fanger) index (scenario PMV). In that case it is assumed that the occupants are turning on and off the cooling system depending on their subjective sense of comfort, as quantified by the PMV index. The control value was set to PMV = 0.

Increased ventilation, with ventilation rates set at 1.5 ACH, and control of the operative temperature (scenario OPT VENT)

Increased insulation, with a thickness of 200 mm, and control of the operative temperature (scenario INS)

Results – Cooling load

Accumulative cooling load

Breakout of heat gains

Cooling load

0

10

20

30

40

50

60

Pu

rch

ase

d C

oo

ling

Rat

e [W

/m2]

Time [h]

473.29, 31%

330.35, 21%

64.08, 4%

675.54, 44%

WINDOW OPAQUE VENTILATION INTERNAL

0

10

20

30

40

50

60

10 20 30 40

Pu

rch

ase

d C

oo

ling

Rat

e [W

/m2]

Ambient Air Temperature [oC]

Results – PMV Index

PMV index in a room (limits for 10% of

dissatisfied people – -0.5≤PMV≤ +0.5)

Results – ASHRAE Standard 55

ASHRAE Standard 55 comfort area (limits

for 10% of dissatisfied people)

Results – Energy & Load

0 250 500 750 1000 1250 1500 1750

PA

FC

RF FC

RF

RFG

RC

RW

Purchased Cooling [kWh]

0 10 20 30 40 50 60

PA

FC

RF FC

RF

RFG

RC

RW

Maximum Purchased Cooling Load [W/m2]

Cooling energy consumption [kWh]

Maximum cooling load [W/m2]

Results – Scenarios

0 250 500 750 1000 1250 1500 1750

PA

FC

RF …

RF

RFG

RC

RW

Purchased Cooling [kWh] - 10 OPT0 250 500 750 1000 1250 1500 1750

PA

FC

RF …

RF

RFG

RC

RW

Purchased Cooling [kWh] - 10 OPT VENT

0 250 500 750 1000 1250 1500 1750

PA

FC

RF …

RF

RFG

RC

RW

Purchased Cooling [kWh] - 10 PMV0 250 500 750 1000 1250 1500 1750

PA

FC

RF …

RF

RFG

RC

RW

Purchased Cooling [kWh] - 20 INS

Discussion

Radiant building surfaces present two disadvantages:

They cannot face the latent loads and

have limited capacity.

Radiant systems offer satisfactory cooling with lower thermal energy consumption only when humidity is not an issue.

The hybrid system was offering for all scenarios considered the best thermal comfort conditions with lower energy consumption than the exclusive air system.

Discussion – The Stochastic Approach

If radiant floors can be assumed as non mechanical cooling systems, due to their extremely discrete operation:

absence of draughts and noise due to fan operation,

very limited radiant temperature asymmetry,

mild surface temperatures and

absence of vertical air temperature differences

then why not be assessed by applying the adaptive comfort models (which are based on the psychological expectation of occupants that in their space there is no artificial cooling offered)?

Discussion – The Stochastic Approach

In the adaptive approach the comfort, and therefore also the operative temperature in buildings cooled by radiant floors can be estimated by:

Tcomf = 0.33Trm + 18.8 (1)

Trm = (1 – α)Tod-1 + αTrm-1 (2)

where,

Trm, the exponentially weighted running mean temperature for a day

α, a constant (= 0.8)

Tod-1, the 24-h daily mean temperatures of the previous day

Trm-1, the exponentially weighted running mean temperature for the previous day

Tcomf, the neutral or comfort temperature (operative temperature at which the average person will be thermally neutral)

Results – The Stochastic Approach

Accepting again 10% dissatisfaction, a ±2 K limit range from the comfort temperature can be determined.

20

22

24

26

28

30

32

15 18 21 24 27 30

Co

mfo

rt (

op

era

tive

) te

mp

era

ture

Tco

mf(o

C)

Running mean outdoor temperature Trm [oC]

10 DBT RF

20

22

24

26

28

30

32

15 18 21 24 27 30

Co

mfo

rt (

op

era

tive

) te

mp

era

ture

Tco

mf(o

C)

Running mean outdoor temperature Trm [oC]

10 OPT RF

THANK YOU